Alkaloids from the Genus Duguetia

Edwin Cardona perez

Diclinanona calycina R. E. Fries popularly known as “envira”, is a species of the Annonaceae family endemic to Brazil. In our ongoing search for bioactive compounds from Annonaceae Amazon plants, the bark of D. calycina was investigated by classical chromatography techniques that yielded thirteen compounds (alkaloids and flavonoids) described for the first time in D. calycina as well as in the genus Diclinanona. The structure of these isolated compounds were established by extensive analysis using 1D/2D-NMR spectroscopy in combination with MS. The isolated alkaloids were identified as belonging to the subclasses: simple isoquinoline, thalifoline (1); aporphine, anonaine (2); oxoaporphine, liriodenine (3); benzyltetrahydroisoquinolines, (S)-(+)-reticuline (4); dehydro-oxonorreticuline (3,4-dihydro-7-hydroxy-6-methoxy-1-isoquinolinyl)(3-hydroxy-4-methoxyphenyl)-methanone) (5); (+)-1S,2R-reticuline Nβ-oxide (6); and (+)-1S,2S-reticuline Nα-oxide (7); tetrahydroprotoberberine, coreximin...

Leishmaniasis, a group of tropical diseases caused by protozoan parasites of genus Leishmania, is a major health problem worldwide that affects millions of people especially in the developing nations. Generic pentavalent antimonials have been the mainstay for therapy in the endemic regions due to efficacy and cost effectiveness, but the growing incidence of their resistance has seriously hampered their use. In many cases the drugs employed for the treatment are toxic, marginally effective, given by injection and, compromised by the development of resistance. Therefore, the development of new mechanism based safe, effective and affordable chemotherapeutic agents to fight leishmaniasis would be an urgent priority research. The recent researches focused on natural products have shown a wise way to get a true and potentially rich source of drug candidates against leishmaniasis, where alkaloids have been found more effective. The present review briefly illustrates an account on current status of leishmaniasis, life cycle of parasites and biology, synergy of the disease with HIV, therapeutic options available to cure this disease and, highlights why natural products especially alkaloids as folk medicines are so important? Additionally, the outlines for the leishmanicidal activities of various alkaloids including indole, quinoline, isoquinoline, pyrimidine-β-carboline, steroidal and diterpene alkaloids from various plants as well as alkaloids from marine sources have been provided with their mechanistic studies.

The aporphine alkaloids constitute the second-largest group of isoquinoline alkaloids. Nevertheless, only a relatively small number of natural aporphines and their derivatives have been studied from a pharmacological viewpoint. Here we review the pharmacological data available for these compounds as related to their dopaminergic, noradrenergic and serotonergic activities, and also some results pertaining to their effects on ion channels and enzymes.

Provided for non-commercial research and educational use only. Not for reproduction, distribution or commercial use. This chapter was originally published in the book The Alkaloids, published by Elsevier, and the attached copy is provided by Elsevier for the author’s benefit and for the benefit of the author’s institution, for non-commercial research and educational use including without limitation use in instruction at your institution, sending it to specific colleagues who know you, and providing a copy to your institution’s administrator. All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institution’s website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier’s permissions site at: http://www.elsevier.com/locate/permissionusematerial From Bruce K. Cassels, Alkaloids from the Genus Duguetia. In: Geoffrey A. Cordell, editors, The Alkaloids. Chennai: Academic Press, 2010, pp. 83-156. ISBN: 978-0-12-381335-0 © Copyright 2010 Elsevier Inc. Academic Press. Author’s personal copy CHAPT ER 3 Alkaloids from the Genus Duguetia Edwin G. Pérez1,3,w and Bruce K. Cassels2,3,* Contents I. Introduction II. Botanical Considerations III. Alkaloids from Chemically Investigated Duguetia species A. Benzyltetrahydroisoquinolines B. Bisbenzyltetrahydroisoquinolines C. Berbines and Protoberberines D. Morphinandienone E. Aporphinoids F. Miscellaneous Aporphinoid- and Berbinoid-Related Alkaloids IV. Structure and Chemistry A. Benzyltetrahydroisoquinolines B. Bisbenzyltetrahydroisoquinoline C. Berbinoids D. Morphinandienone E. Aporphinoids F. Miscellaneous Aporphinoid- and Berbinoid-Related Alkaloids V. Biosynthesis, Biogenesis, and Chemosystematics VI. Ethnopharmacology and Pharmacology A. Benzyltetrahydroisoquinolines B. Bisbenzylisoquinoline C. Berbinoids 1 Department of Chemistry, Faculty of Chemistry and Biology, University of Santiago, Santiago, Chile 2 Department of Chemistry, Faculty of Sciences, University of Chile, Chile 3 w 84 84 86 106 107 107 109 109 114 117 117 117 117 117 117 119 122 132 133 134 134 Millennium Institute for Cell Dynamics and Biotechnology, Santiago, Chile Present address: Facultad de Quı́mica, Pontificia Universidad Católica de Chile, Santiago, Chile * Corresponding author. E-mail address: bruce.cassels@gmail.com (B.K. Cassels) The Alkaloids, Volume 68 ISSN: 1099-4831, DOI 10.1016/S1099-4831(10)06803-3 r 2010 Elsevier Inc. All rights reserved 83 Author’s personal copy 84 Edwin G. Pérez and Bruce K. Cassels D. Protoberberines E. Glaziovine F. Aporphines G. Oxoaporphines H. Aminoethylphenanthrenes I. Copyrine Alkaloids J. 1-Aza-9,10-anthraquinones VII. Concluding Remarks Acknowledgments References 137 139 139 143 144 145 146 146 147 147 I. INTRODUCTION Duguetia A. St.-Hil. (Annonaceae) is a genus of usually small, understory trees growing almost exclusively in the tropics of South America, with a small extension across the Panama Isthmus. It is now regarded as comprising close to 100 species, considering the recent inclusion of four African taxa, of which three were previously known as Pachypodanthium Engler & Diels. It is therefore one of the largest Annonaceous genera after Guatteria and Annona. Many studies have been conducted on the secondary metabolites present in different parts of Duguetia plants, from which essential oils, aromatic compounds, monoterpenes, diterpenes, triterpenes, flavonoids, and most typically alkaloids have been isolated and characterized. In common with the other ‘‘primitive angiosperms,’’ Duguetia species accumulate isoquinoline alkaloids, and more specifically 1-benzyl-1,2,3,4-tetrahydroisoquinolines, usually referred to simply as ‘‘benzylisoquinolines,’’ and their biosynthetic or biogenetically presumed derivatives. The literature reports studies on the alkaloids of about 16 Duguetia species (one of which was not clearly identified), resulting in the isolation and identification or characterization of 105 different alkaloids. Although many of these alkaloids are widely distributed, a few unusual groups of alkaloids appear to be specific to this genus. II. BOTANICAL CONSIDERATIONS The plants of the Annonaceae have traditionally been classed as part of the order Magnoliales. In the most recent consensus, the Magnoliales and Laurales constitute one of the two sister clades in the Magnoliidae, which are commonly regarded as the most ‘‘primitive’’ angiosperms in older classifications (1,2). Author’s personal copy Alkaloids from the Genus Duguetia 85 Regarding the occurrence of benzylisoquinoline alkaloids in the Annonaceae, other magnoliids, and more distantly related families, it is of interest to note that there is now good biochemical and molecular phylogenetic evidence for the evolution of benzylisoquinoline alkaloid biosynthesis in angiosperms from a common ancestor. Activity ascribable to the first enzyme in this biosynthetic tree, (S)-norcoclaurine synthase, occurs in 90 different plant species, and compares well with a molecular phylogeny. Phylogenetic analyses of norcoclaurine synthase, the berberine bridge enzyme, and several O-methyltransferases ‘‘suggest a latent molecular fingerprint for benzylisoquinoline alkaloid biosynthesis in angiosperms not known to accumulate such alkaloids’’ (3). Duguetia was thought, on the basis of inflorescence and floral characters, to form an alliance with the very small neotropical genera Duckeanthus, Fusaea, and Malmea, and the African Letestudoxa (4). The monotypic Pseudartabotrys was later included and Malmea excluded (5), but incorporation of leaf, flower, fruit, and seed characters that had not been considered previously has led to a different grouping in which Duguetia (including Pachypodanthium) constitutes a clade of its own, close to a separate sister group including Fusaea, Duckeanthus, Letestudoxa, and Pseudartabotrys (6). Despite the inclusion of Pachypodanthium as ‘‘African species of Duguetia,’’ these plants still form a small, distinct cluster, perhaps not surprisingly together with Duguetia riberensis of Venezuela, in this cladistic analysis. The genus has been further subdivided into 14 sections by Fries based on their morphological characters, but leaving some species in uncertain positions (7,8). These subdivisions have largely been upheld by a more recent study (9), and it is the system used in this review (Table I). One third of all Duguetia species were analyzed in a study based on their genomic DNA sequences (41). That work supported the notion that Duguetia, like Guatteria, is monophyletic, with its most recent common ancestor dating back to 29.0474.52 million years ago (in the case of Guatteria this figure is 36.6572.50 mybp), although the authors concede that ‘‘the accuracy of the absolute dates remains unassessed.’’ A fossilized leaf from the middle Eocene period (about 38 48 mybp) from Western Tennessee, when the local climate was subtropical to tropical, has been classified as belonging to a Duguetia species (42), a conclusion that seems to conflict with the estimated DNA age of the genus. On the basis of its present geographic, trans-Atlantic distribution it was suggested that the Duguetia clade might predate the break-up of Gondwana (6). As the separation of Africa and South Author’s personal copy 86 Edwin G. Pérez and Bruce K. Cassels America is believed to have been completed in the early Cretaceous (about 110 million years ago), and the age of the Annonaceae as a family is estimated to be as little as 82 million years (43), it seems necessary to assume long-distance dispersal over the widening early Atlantic Ocean, possibly across stepping-stones along the 80 million-year-old volcanic Sierra Leone Rise (to which the Ceará Rise should be added) (44) or, less likely, the more southerly Walvis Ridge (and Rio Grande Rise) (45). This hypothesis seems reasonable given the presence of Annonaceae in the Lesser Antilles, which would represent much more recent (Pliocene or even Pleistocene) events of a similar character (46). III. ALKALOIDS FROM CHEMICALLY INVESTIGATED DUGUETIA SPECIES The Duguetia species studied to date for their alkaloidal content are listed in Table I, ordered by sections, and in alphabetical order when appropriate. All of the alkaloids isolated from this genus have at least a formal isoquinoline-derived structure; including the 1-azaanthraquinone cleistopholine and the rare copyrine alkaloids, the 1-aza-7oxoaporphines and 1-aza-4,5-dioxoaporphines. These alkaloids are classified as benzyltetrahydroisoquinolines, a single bisbenzyltetrahydroisoquinoline, berbines (tetrahydroprotoberberines), protoberberines, a morphinandienone, a proaporphine, and many aporphinoids and aporphinoid-related compounds. A large proportion of the aporphines are oxygenated at C7, a fairly common feature in the Annonaceae. 7-Methoxy derivatives are almost completely restricted to the African Duguetia species. Four N-formylnoraporphines have been identified. Three nitroso- or nitroaporphinoid derivatives isolated from Duguetia furfuracea might be artifacts, as discussed below. Several of the aporphinoids have the unusual 9,11-dioxygenation pattern in ring D which, aside from Duguetia, has only been found in one Guatteria species. As in Guatteria, some of the Duguetia aporphinoids bear a biogenetically intriguing carbon atom bonded to C7. Finally, a protoberberine styrene adduct is a unique alkaloid from the African Duguetia staudtii. Table II lists the 105 alkaloids, including some possible artifacts, ordered according to their main structural features, as depicted in Figure 1 (Table III). In many cases, the structures were known prior to their isolation from Duguetia species, or were very closely related to known alkaloids, Author’s personal copy Table I Chemically investigated Duguetia species and their contained alkaloids Alkaloid Duguetia R. E. Fries D. furfuracea (A. St.-Hil.) Benth. & Hook. Reticuline Isochondodendrine Discretamine Isocorydine Norisocorydine Xylopine Obovanine Anonaine Asimilobine Atherospermidine Liriodenine Lanuginosine Duguetine N-Oxyduguetine Dicentrinone N-Methylglaucine N-Methyl-tetrahydropalmatine N-Nitrosoanonaine N-Nitrosoxylopine 8-Nitroisocorydine Dehydrodiscretine Pseudopalmatine Oliveroline N-Methylguatterine D. odorata (Diels) J. F. Macbr. Structure 1 3 4 41 40 28 30 23 20 86 83 87 76 77 91 36 8 51 52 42 16 17 60 66 Ref.(s) 10 10 10 10 10 10 10 10 10 10 10 10 11 11 11 11 11 12 12 13 14 14 14 14 87 Species Alkaloids from the Genus Duguetia Section Author’s personal copy 88 Table I (Continued) Section Alkaloid Structure Ref.(s) D. stelechantha (Diels) R. E. Fries Oxopukateine O-Methylmoschatoline Corypalmine Hadranthine A Hadranthine B Imbiline-1 Sampangine 3-Methoxysampangine Discretamine 10-Demethylxylopinine Xylopine Puterine O-Methylpukateine Obovanine Oxoputerine Atherosperminine Calycinine Noratherosperminine Duguecalyne N-Formylputerine Duguenaine Xylopine Isolaureline 88 85 5 99 100 101 97 98 4 11 28 31 32 30 89 94 43 93 54 53 47 28 29 15 15 15 16 16 16 16 16 17 17 17 17 17 17 17 17 17 18 19 19 20 20 20 Hadrantha R. E. Fries D. hadrantha (Diels) R. E. Fries Sphaerantha R. E. Fries D. calycina Benoist D. obovata R. E. Fries Edwin G. Pérez and Bruce K. Cassels Species Author’s personal copy 48 33 34 49 27 43 44 45 50 46 90 12 10 18 2 21 67 68 70 56 58 59 73 74 71 72 89 20 20 20 20 20 20 20 20 20 20 20 20 20 20 21,22 21 21 21 21 21 21 21 21 21 21 21 Alkaloids from the Genus Duguetia D. spixiana Mart. (Colombia) N-Formylxylopine Buxifoline N-Methylbuxifoline N-Formylbuxifoline Anolobine Calycinine N-Methylcalycinine Duguevanine N-Formylduguevanine N-Methylduguevanine Oxobuxifoline Xylopinine Discretine (9S)-Sebiferine N-Oxycodamine N-Methylasimilobine Noroliveridine Oliveridine N-Oxyoliveridine Norpachyconfine Pachyconfine N-Oxypachyconfine Spixianine N-Oxyspixianine Duguexine N-Oxyduguexine Author’s personal copy 90 Table I (Continued) Section Species Structure Ref.(s) Lanuginosine Atherosperminine N-Oxyatherosperminine Methoxyatherosperminine Spiduxine Duguespixine Anonaine Nornuciferine 3-Hydroxynornuciferine O-Methylisopiline Noroliveridine Oliveridine N-Oxyoliveridine Duguexine Roemerolidine Nornuciferidine Rurrebanine Rurrebanidine Lysicamine Lanuginosine O-Methylmoschatoline Spiguetidine 87 94 95 96 13 55 23 22 25 26 67 68 70 71 69 57 63 62 84 87 85 103 21 21 21 21 21 21,23 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 Edwin G. Pérez and Bruce K. Cassels D. spixiana Mart. (Bolivia) Alkaloid Author’s personal copy D. vallicola J. F. Macbr. Polyantha R. E. Fries D. eximia Diels Geanthemum R. E. Fries D. flagellaris Huber 102 12 7 37 41 38 68 60 92 85 12 10 17 104 19 85 88 89 22 24 26 43 45 78 60 24 24 24 25 26 26 27 27 27 27 26 26 26 27 26 28 28 28 29,30 29,30 29,30 29,30 29,30 29,30 29,30 Alkaloids from the Genus Duguetia Calothrix R. E. Fries Spiguetine Xylopinine Tetrahydropalmatine N-Methyllaurotetanine Isocorydine Isoboldine Oliveridine Oliveroline Duguevalline O-Methylmoschatoline Xylopinine Discretine Pseudopalmatine Cleistopholine Glaziovine O-Methylmoschatoline Oxopukateine Oxoputerine Nornuciferine Isopiline O-Methylisopiline Calycinine Duguevanine Pachypodanthine Oliveroline 91 Author’s personal copy 92 Table I (Continued) Section D. colombiana Maas D. gardneriana Mart. D. glabriuscula R. E. Fries D. magnolioidea Maas D. trunciflora Maas Undetermined Duguetia sp. African species D. confinis (Engl. & Diels) Chatrou Alkaloid N-Oxyoliveroline Oliveridine Duguetine O-Methylmoschatoline Discretamine Corypalmine Tetrahydropalmatine Polyalthine Oliveridine Oxobuxifoline Lanuginosine Discretamine Reticuline Tetrahydropalmatine Corypalmine Discretamine Thaicanine Jatrorrhizine Norglaucine Dicentrine Duguetine Corypalmine Isocorypalmine Structure 61 68 76 85 4 5 7 75 68 90 87 4 1 7 5 4 14 15 35 39 76 5 6 Ref.(s) 29,30 29,30 29,30 31 32 32 32 33 33 33 33 34 35 35 35 35 35 35 36 36 36 37 37 Edwin G. Pérez and Bruce K. Cassels Uncertain Species Author’s personal copy D. staudtii (Engl. & Diels) Chatrou 7 9 10 60 64 65 58 78 80 5 6 10 79 82 81 83 105 78 37 38 38 37 37 37 37 38 38 39 40 39 39 39 39 39,40 39,40 39,40 Alkaloids from the Genus Duguetia Tetrahydropalmatine Govanine Discretine Oliveroline Guatterine N-Oxyguatterine Pachyconfine Pachypodanthine N-Acetylpachypodanthine Corypalmine Isocorypalmine Discretine N-Methylpachypodanthine Pachystaudine Norpachystaudine Liriodenine Staudine Pachypodanthine 93 Author’s personal copy 94 Table II Alkaloids isolated from Duguetia species Alkaloid type and name MW Species Ref.(s) 1 C19H23NO4 329 cis-N-Oxycodamine 2 C20H25NO5 359 D. furfuracea D. trunciflora D. spixianaa 10 35 21,22 Bisbenzylisoquinoline Isochondodendrine 3 C36H28N2O6 594 D. furfuracea 10 Berbines (Tetrahydroprotoberberines) ( )-Discretamine 4 C19H21NO4 327 ( )-Corypalmine (Tetrahydrojatrorrhizine) 5 C20H23NO4 341 ( )-Isocorypalmine 6 C20H23NO4 341 ( )-Tetrahydropalmatine (Rotundine) 7 C21H25NO4 355 D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. 17 32 10 35 34 32 15 35 39 37 40 37 37 24 32 35 calycina gardneriana furfuracea trunciflora magnolioidea gardneriana stelechantha trunciflora staudtii confinis staudtii confinis confinis spixianab gardneriana trunciflora Edwin G. Pérez and Bruce K. Cassels Molecular formula Benzylisoquinolines (þ)-Reticuline Structure Author’s personal copy C22H28NO4 C20H23NO4 C20H23NO4 370 341 341 D. D. D. D. D. D. D. D. D. D. D. D. furfuracea confinis obovata vallicola confinis staudtii calycina obovata spixianab vallicola spixianaa trunciflora 11 38 20 26 38 39 17 20 24 26 21 35 ( )-10-Demethylxylopinine ( )-Xylopinine 11 12 C20H23NO4 C21H25NO4 341 355 ( )-Spiduxine ( )-Thaicanine 13 14 C21H23NO5 C21H25NO5 369 371 Protoberberines Jatrorrhizine Dehydrodiscretine Pseudopalmatine 15 16 17 C20H20NO4 C20H20NO4 C21H22NO4 338 338 352 D. D. D. D. trunciflora odorata odorata vallicola 35 14 14 26 Morphinandienone (9S)-Sebiferine 18 C20H23NO4 341 D. obovata 20 Proaporphine ( )-Glaziovine 19 C18H19NO3 297 D. vallicola 26 Aporphines sensu stricto Asimilobine N-Methylasimilobine 20 21 C17H17NO2 C18H19NO2 267 281 D. furfuracea D. spixianaa 10 21 95 8 9 10 Alkaloids from the Genus Duguetia N-Methyltetrahydropalmatine ( )-Govanine ( )-Discretine Author’s personal copy 96 Table II (Continued) Alkaloid type and name Structure MW Species Ref.(s) D. spixianab D. flagellaris D. spixianab D. furfuracea D. flagellaris D. spixianab D. spixianab D. flagellaris D. obovata D. calycina D. obovata D. furfuracea D. obovata D. calycina D. furfuracea D. calycina D. calycina D. obovata D. obovata Duguetia sp. D. furfuracea D. vallicola D. vallicola 24 29,30 24 10 29,30 24 24 29,30 20 17 20 10 20 17 10 17 17 20 20 36 11 25 26 Nornuciferine 22 C18H19NO2 281 Anonaine 23 C17H15NO2 265 Isopiline 3-Hydroxynornuciferine O-Methylisopiline 24 25 26 C18H19NO3 C18H19NO3 C19H21NO3 297 297 311 Anolobine Xylopine 27 28 C17H15NO3 C18H17NO3 281 295 Isolaureline Obovanine 29 30 C19H10NO3 C17H15NO3 309 281 Puterine O-Methylpukateine Buxifoline N-Methylbuxifoline Norglaucine N-Methylglaucine N-Methyllaurotetanine Isoboldine 31 32 33 34 35 36 37 38 C18H17NO3 C19H19NO3 C19H19NO4 C20H21NO4 C20H23NO4 C22H28NO4 C20H23NO4 C19H21NO4 295 309 325 339 341 370 341 327 Edwin G. Pérez and Bruce K. Cassels Molecular formula Author’s personal copy 36 10 10 26 13 17 29,30 20 20 20 29,30 20 C19H17NO4 C19H17NO4 C20H19NO5 C20H21NO6 323 323 353 369 D. D. D. D. calycina obovata obovata obovata 19 20 20 20 51 52 C17H14N2O3 C18H16N2O4 294 324 D. furfuracea D. furfuracea 12 12 53 54 55 C19H15NO3 C20H17NO4 C19H17NO3 305 335 307 D. calycina D. calycina D. spixianaa 20 19 21,23 39 40 41 C20H21NO4 C19H21NO4 C20H23NO4 339 327 341 8-Nitroisocorydine Calycinine 42 43 C20H22N2O6 C18H17NO4 386 311 N-Methylcalycinine Duguevanine 44 45 C19H19NO4 C19H19NO5 325 341 N-Methylduguevanine 46 C20H21NO5 N-Formylnoraporphines N-Formylputerine N-Formylxylopine N-Formylbuxifoline N-Formylduguevanine 47 48 49 50 N-Nitrosonoraporphines N-Nitrosoanonaine N-Nitrosoxylopine 7-Alkyl-substituted-6a,7-dehydroaporphines Duguenaine Duguecalyne Duguespixine Alkaloids from the Genus Duguetia 355 Duguetia sp. D. furfuracea D. furfuracea D. vallicola D. furfuracea D. calycina D. flagellaris D. obovata D. obovata D. obovata D. flagellaris D. obovata Dicentrine Norisocorydine Isocorydine 97 Author’s personal copy 98 Table II (Continued) Alkaloid type and name MW Species Ref.(s) 56 57 58 C17H17NO3 C18H19NO3 C18H19NO3 283 297 297 N-oxypachyconfine Oliveroline 59 60 C18H19NO4 C18H17NO3 313 295 N-Oxyoliveroline Rurrebanidine Rurrebanine Guatterine N-Oxyguatterine N-Methylguatterine Noroliveridine Oliveridine 61 62 63 64 65 66 67 68 C18H17NO4 C18H19NO4 C19H21NO4 C19H19NO4 C18H17NO5 C20H22NO4 C17H15NO3 C19H19NO4 311 313 327 325 341 340 281 325 Roemerolidine N-Oxyoliveridine 69 70 C18H17NO4 C19H19NO5 311 341 D. spixianaa D. spixianab D. confinis D. spixianaa D. spixianaa D. confinis D. flagellaris D. vallicola D. odorata D. flagellaris D. spixianab D. spixainab D. confinis D.confinis D. odorata D. spixianaa,b D. spixianaa,b D. glabriuscula D. flagellaris D. vallicola D. spixianab D. spixianaa,b 21 24 37 21 21 37 29,30 27 14 29,30 24 24 37 37 14 21,24 21,24 33 29,30 27 24 21,24 Edwin G. Pérez and Bruce K. Cassels Molecular formula 7-Hydroxyaporphines Norpachyconfine Nornuciferidine Pachyconfine Structure Author’s personal copy 21,24 21 21 21 33 36 29,30 11 11 309 337 D. D. D. D. D. 39,47 38 29,30 39 38 C18H17NO4 C19H19NO4 311 235 D. staudtii D. staudtii 39 39 83 C17H9NO3 275 84 85 C18H13NO3 C19H15NO4 291 321 D. D. D. D. D. 10 39,40 24 24 15 71 72 73 74 75 76 C18H17NO4 C18H17NO5 C19H19NO5 C19H19NO6 C20H21NO5 C20H21NO5 311 327 341 357 355 355 N-Oxyduguetine 77 C20H21NO6 371 78 C18H17NO3 295 N-Methylpachypodanthine N-Acetylpachypodanthine 79 80 C19H19NO3 C20H19NO4 7-Methoxy-4-hydroxyaporphines Norpachystaudine Pachystaudine 81 82 Oxoaporphines Liriodenine 7-Methoxyaporphines Pachypodanthine Lysicamine O-Methylmoschatoline staudtii confinis flagellaris staudtii confinis furfuracea staudtii spixianab spixianab stelechantha Alkaloids from the Genus Duguetia D. spixianaa,b D. spixianaa D. spixianaa D. spixianaa D. glabriuscula Duguetia sp. D. flagellaris D. furfuracea D. furfuracea Duguexine N-Oxyduguexine Spixianine N-Oxyspixianine Polyalthine Duguetine 99 Author’s personal copy 100 Table II (Continued) Alkaloid type and name Structure Molecular formula MW 86 87 C18H11NO4 C18H11NO4 305 305 Oxopukateine 88 C17H9NO4 291 Oxoputerine 89 C18H11NO4 305 Oxobuxifoline 90 C19H13NO5 335 Dicentrinone Duguevalline 91 92 C19H13NO5 C20H15NO6 335 365 93 94 C19H21NO2 C20H23NO2 295 309 95 96 C22H23NO3 C21H25NO3 325 339 Aminoethylphenanthrenes (6,6a-Secoaporphines) Noratherosperminine Atherosperminine N-Oxyatherosperminine Methoxyatherosperminine Ref.(s) D. D. D. D. D. D. D. D. D. D. D. D. D. D. D. eximia vallicola colombiana furfuracea glabriuscula furfuracea spixianaa,b eximia stelechantha eximia calycina obovata glabriuscula furfuracea vallicola 28 27 31 10 33 10 21,24 28 15 28 17 20 33 11 27 D. D. D. D. D. calycina spixianaa calycina spixianaa spixianaa 18 21 17 21 21 Edwin G. Pérez and Bruce K. Cassels Atherospermidine Lanuginosine Species Author’s personal copy 97 98 C15H8N2O C16H10N2O2 232 246 D. hadrantha D. hadrantha 16 16 1-Aza-4,5-dioxoaporphines Hadranthine A Hadranthine B Imbiline-1 99 100 101 C18H14N2O4 C16H10N2O3 C17H12N2O3 322 278 292 D. hadrantha D. hadrantha D. hadrantha 16 16 16 Azahomoaporphines Spiguetine Spiguetidine 102 103 C18H16N2O3 C19H18N2O3 308 322 D. spixianab D. spixianab 24 24 Azaanthraquinone Cleistopholine 104 C14H9NO2 223 D. vallicola 27 Protoberberine styrene adduct Staudine 105 C31H33NO7 531 D. staudtii 39,48 D. spixiana from Colombia. D. spixiana from Bolivia. b Alkaloids from the Genus Duguetia a Copyrine alkaloids 1-Aza-7-oxoaporphines Sampangine 3-Methoxysampangine 101 Author’s personal copy 102 Table III Alphabetical list of alkaloids isolated from the genus Duguetia with their synonyms and structure numbers Name Structure Name Dehydrodiscretine ( )-10-Demethylxylopinine Dicentrine (N,O-Dimethylactinodaphnine, Eximine) Dicentrinone ( )-Discretamine ( )-Discretine Duguecalyne Duguenaine Duguespixine Duguetine Duguevalline 16 11 39 Imbiline-1 Isoboldine (N-Methyllaurelliptine) Isochondodendrine Isocorydine (Artabotrine, Luteanine) ( )-Isocorypalmine Isolaureline (N-Methylxylopine) Isopiline Jatrorrhizine Lanuginosine (Oxoxylopine) Liriodenine (Oxoushinsunine, Micheline B, Spermatheridine) Lysicamine (Oxonuciferine) Methoxyatherosperminine 3-Methoxysampangine 91 4 10 54 53 55 76 92 N-Methylasimilobine N-Methylbuxifoline N-Methylcalycinine N-Methylduguevanine N-Methylglaucine N-Methylguatterine O-Methylisopiline (O-Methylnorlirinine) N-Methyllaurotetanine (Lauroscholtzine, Rogersine) 101 38 3 41 6 29 24 15 87 83 84 96 98 21 34 44 46 36 66 26 37 Edwin G. Pérez and Bruce K. Cassels N-Acetylpachypodanthine 80 Anolobine 27 Anonaine 23 Asimilobine 20 Atherospermidine (Psilopine) 86 Atherosperminine 94 Buxifoline 33 Calycinine 43 Cleistopholine 104 ( )-Corypalmine (Tetrahydrojatrorrhizine) 5 Structure Author’s personal copy O-Methylmoschatoline (Liridine, Homomoschatoline) 85 N-Methylpachypodanthine 79 O-Methylpukateine 32 N-Methyltetrahydropalmatine 8 8-Nitroisocorydine 42 N-Nitrosoanonaine 51 N-Nitrosoxylopine 52 Noratherosperminine 93 Norglaucine 35 Norisocorydine 40 Nornuciferidine 57 Nornuciferine 22 Pachystaudine 82 Polyalthine 75 Pseudopalmatine 17 Puterine 31 (þ)-Reticuline 1 Roemerolidine 69 Rurrebanidine 62 Rurrebanine 63 Sampangine 97 (9S)-Sebiferine 18 ( )-Spiduxine 13 Spiguetidine 103 Spiguetine 102 Spixianine 73 103 45 71 49 50 47 48 10 9 64 99 100 25 67 56 81 30 68 60 90 88 89 95 2 77 72 65 Alkaloids from the Genus Duguetia Duguevanine Duguexine N-Formylbuxifoline N-Formylduguevanine N-Formylputerine N-Formylxylopine ( )-Glaziovine ( )-Govanine Guatterine Hadranthine A Hadranthine B 3-Hydroxynornuciferine Noroliveridine Norpachyconfine Norpachystaudine Obovanine Oliveridine Oliveroline Oxobuxifoline Oxopukateine Oxoputerine N-Oxyatherosperminine cis-N-Oxycodamine N-Oxyduguetine N-Oxyduguexine N-Oxyguatterine Author’s personal copy 104 Table III (Continued) Name 70 61 59 74 58 78 Staudine ( )-Tetrahydropalmatine (Rotundine) ( )-Thaicanine Xylopine (O-Methylanolobine) ( )-Xylopinine Structure 105 7 14 28 12 Edwin G. Pérez and Bruce K. Cassels N-Oxyoliveridine N-Oxyoliveroline N-Oxypachyconfine N-Oxyspixianine Pachyconfine Pachypodanthine Structure Name Author’s personal copy 105 Alkaloids from the Genus Duguetia and in other instances the structure elucidations were straightforward, relying largely on the NMR spectra of the alkaloids. For this reason, in this section only the more problematic structure assignments will be discussed. Benzylisoquinoline type Berbine type Bisbenzylisoquinoline type Isochondodendrine subtype N CH3 8´ 12 O R Protoberberine type N R´ 1´ N 8 O 1 12´ N Morphinandienone type Proaporphine type N N R N R O O Aporphine type Aporphine sensu stricto subtype N-Formylnoraporphine subtype N-Nitrosonoraporphine subtype 4 5 2 N 1 N CH3 O N N O H 7 10 9 Duguenaine subtype N Duguespixine subtype N O 7-Hydroxyaporphine subtype H O CH3 Figure 1 Structural types of alkaloids isolated from Duguetia species. N R OH Author’s personal copy 106 Edwin G. Pérez and Bruce K. Cassels 7-Oxoporphine subtype Aminoethylphenanthrene subtype R R N N O COPYRINE ALKALOIDS 1-Aza-7-oxoaporphine subtype 1-Aza-4,5-dioxoaporphine subtype O O N N N N R O Azahomoaporphine type 1-Azaanthraquinone type O N CH3 CH3 N N O Figure 1 (Continued) A. Benzyltetrahydroisoquinolines Only two, unelaborated, benzyltetrahydroisoquinolines have been reported from the genus Duguetia, namely, reticuline (1), isolated from Duguetia trunciflora and D. furfuracea (10,35), and cis-N-oxycodamine (2), isolated from Duguetia spixiana (21,22). 5 H3CO 6 4 3 N2 1 CH3 7 HO HO 2´ 4´ H3CO H3CO N HO H3CO 6´ H3CO 1 2 O CH3 Author’s personal copy Alkaloids from the Genus Duguetia 107 B. Bisbenzyltetrahydroisoquinolines The head-to-tail/head-to-tail dimer isochondodendrine (3), isolated from D. furfuracea (10), is the only bisbenzyltetrahydroisoquinoline recorded to date from this genus. H3CO N HO CH3 O H3C O N OH OCH3 3 C. Berbines and Protoberberines The berbines or tetrahydroprotoberberines appear to be widely distributed in the genus Duguetia (10 out of 15 16 species studied). Although quantitative analyses are lacking, it is noteworthy that these alkaloids comprise more than 50% of the mass of alkaloids isolated from the bark of the African D. confinis, and about 20% of D. staudtii, while they are apparently less abundant in the New World species. It is also noteworthy that, aside from their common precursor reticuline (1), the other five alkaloids isolated from D. trunciflora are members of this structural type, as do all three Duguetia gardneriana alkaloids. With the exception of spiduxine (13, only known so far from D. spixiana) and thaicanine (14, from D. trunciflora, but isolated previously from other, non-Annonaceous species), their structures are quite commonplace. A single paper on the constituents of D. trunciflora reported the presence of reticuline (1), the berbines tetrahydropalmatine (THP) (rotundine) (7), tetrahydrojatrorrhizine (corypalmine) (5), discretamine (4), and thaicanine (14), and the protoberberine jatrorrhizine (15) (29). Although the optical rotations of the chiral members of this series were not published, all four berbines can be expected to have the usual S-configuration, and the same is true for the reticuline (1) isolated from this plant, if it is the biosynthetic precursor of the other isolates, and not, in this case, a dead-end metabolite with the R stereochemistry. A report on the hypotensive and vasorelaxant effects of discretamine (4) from Duguetia magnolioidea Maas (34) refers to experimental details of the isolation ‘‘according to the method described by Fechine et al. (2002)’’ (35). Unfortunately, the report provides no information as to the location Author’s personal copy 108 Edwin G. Pérez and Bruce K. Cassels where the plant was collected, its identification, or the existence of a voucher specimen. In this genus, the quaternary N-methyltetrahydropalmatine (8) has only been isolated from D. furfuracea. Although its putative precursor, THP (also named rotundine, 7) has not been reported from this species, its 3,10-dihydroxy analog discretamine (4) is present in D. furfuracea, D. calycina, D. gardneriana, D. magnolioidea, and D. trunciflora (10,17,32,34,35). R3O R2O N H3CO 3 H3CO 2 N CH3 OCH3 OR10 11 4 : R2 = CH3, R3= R10 = H 5 : R2 = R10 = CH3, R3= H 6 : R2 = H, R3= R10 = CH3 7 : R2 = R3= R10 = CH3 9 OCH3 10 OCH3 8 R3O R2O N OR10 OCH3 9 : R2 = H, R3=R10 = CH3 10 : R2 = R10 = CH3, R3= H 11 : R2 = R3= CH3, R10 = H 12 : R2 = R3= R10 = CH3 Protoberberines, easily formed nonenzymatically on prolonged exposure of berbines to air, have been isolated less often from Duguetia, but the co-occurrence of jatrorrhizine (15) and its tetrahydro analog corypalmine (5¼tetrahydrojatrorrhizine) in D. trunciflora, and of pseudopalmatine (17) and the corresponding xylopinine (12) in Duguetia vallicola suggest that at least in these species they might be artifacts of storage or isolation. Thaicanine (14) is presumably a hydroxylation metabolite of THP (7). The C12-formylated spiduxine (13) from Author’s personal copy Alkaloids from the Genus Duguetia 109 Colombian D. spixiana is viewed as a (tetrahydro)retroprotoberberine (see Section V). R3O HO N H3CO H3CO N OCH3 OCH3 OCH3 16 : R3 = H 17 : R3 = CH3 OCH3 15 D. Morphinandienone (9S)-Sebiferine (18) is the only morphinandienone reported from this genus, as a constituent of Duguetia obovata (20). OCH3 H3CO N CH3 H3CO O 18 E. Aporphinoids 1. Proaporphines Proaporphines, like the morphinandienones, seem to be uncommon in Duguetia. Only glaziovine (19) has been reported from the leaves of D. vallicola in which it is quite abundant (26). H3CO N HO O 19 CH3 Author’s personal copy 110 Edwin G. Pérez and Bruce K. Cassels 2. Aporphines sensu stricto Aporphinoids in general are richly represented in the genus Duguetia. Aporphines sensu stricto 43 46, N-formylduguevanine (50), the 7-hydroxyaporphines (73 74), and the oxoaporphine duguevalline (92), present the unusual 9,11-dioxygenation pattern. OR3 2 RO H3CO O N H3CO R6 NH O 20 : R 2 = R6 = H 21 : R2 = H, R6 = CH3 22 : R 2 = R6 = CH3 NH R1O 24 : R 1 = H, R3 = CH3 25 : R 1 = CH3, R3 = H 26 : R 1 = R3 = CH3 23 OCH3 O O N O R6 O N O R6 N O R6 11 R O OR9 OCH3 27 : R6 = R9 = H 28 : R6 = H, R9 = CH3 29 : R6 = R9 = CH3 H3CO 30 : R6 = R11 = H 31 : R6 = H, R11 = H 32 : R6 = H, R11 = CH3 H3CO NH H3CO H3CO H3CO CH3 N CH3 H3CO H3CO OCH3 35 33 : R6 = H, 34 : R6 = CH3 N R1O R6 H3CO OCH3 36 OR9 37 : R 1 = R6 = CH3, R9 = H 38 : R 1 = R9 = H, R6 = CH3 Author’s personal copy 111 Alkaloids from the Genus Duguetia H3CO O N O H3CO HO CH3 H3CO N R6 H3CO H3CO N H3CO HO H3CO CH3 NO2 OCH3 40 : R 6 = H 41 : R 6 = CH3 39 42 OCH3 O O N O R6 N O HO R6 HO OCH3 OCH3 6 45 : R 6 = H 46 : R 6 = CH3 43 : R = H 44 : R 6 = CH3 3. N-Formylaporphines Four N-formylaporphines have been reported from the genus Duguetia, namely, N-formylputerine (47) from D. calycina (19), and N-formylxylopine (48), N-formylbuxifoline (49), and N-formylduguevanine (50) from D. obovata (20). O O N O H3CO C H O N O C H O OCH3 47 48 OCH3 OCH3 O O N O C H O N O HO OCH3 OCH3 49 50 C H O Author’s personal copy 112 Edwin G. Pérez and Bruce K. Cassels 4. N-Nitrosoaporphines Two N-nitrosonoraporphines, N-nitrosoanonaine (51) and N-nitrosoxylopine (52) have been reported from D. furfuracea. The structure of N-nitrosoanonaine (51) was confirmed by X-ray crystallography (12). The same authors have very recently reported the presence of 8-nitroisocorydine (42) in the same plant (13). O O N O N O N O N O OCH3 51 52 5. 7-Alkyl-6a,7-didehydroaporphines Duguenaine (53) and duguecalyne (54) were isolated from D. calycina (19,20), and duguespixine (55) from the bark of the Colombian D. spixiana (21,23). The latter alkaloid was also found in Guatteria sagotiana (49), but to date duguecalyne (54) and duguenaine (53) seem to be exclusively Duguetia metabolites. O HO O N O O 53 N O H3CO N H3CO O CH3 O 54 H 55 6. 7-Hydroxyaporphines The genus Duguetia is remarkably rich in 7-hydroxylated aporphines, of which a small number have also been isolated from Guatteria species. Although only found in one half of the species studied, they account for nearly two thirds of the mass of alkaloids isolated from both Colombian and Bolivian D. spixiana. Author’s personal copy Alkaloids from the Genus Duguetia O O O N CH3 H OH O 113 O N CH3 H OH O HO O N CH3 H OH O HO OCH3 71 70 72 OCH3 O O O N CH3 H OH O HO O N CH3 H OH O HO N CH3 H OH O OCH3 OCH3 OCH3 73 74 75 O O N CH3 H OH O H3CO O N CH3 H OH O H3CO OCH3 OCH3 76 77 The closely related pachypodanthine (78), N-methylpachypodanthine (79), N-acetylpachypodanthine (80), pachystaudine (82), and norpachystaudine (81), all C7 methoxylated, are characteristic of the African species D. staudtii and D. confinis (formerly designated as Pachypodanthium). Although a few other C4 C7 oxygenated aporphines (e.g., stephadiolamine) and oxoaporphines are known, pachystaudine (82) and its nor-analog 81 seem to be the only aporphinoids characterized to date with both C4 hydroxy and C7 methoxy substituents. OH O O O N 6 R H OCH3 78: R 6 = H 79: R 6 = CH3 80: R 6 = COCH3 O N 6 R H OCH3 81: R 6 = H 82: R 6 = CH3 Author’s personal copy 114 Edwin G. Pérez and Bruce K. Cassels 7. Oxoaporphines Nine 7-oxoaporphine alkaloids (7-oxo-4,5,6,6a-tetradehydroaporphines) have been isolated from Duguetia species, scattered throughout the genus. Perhaps significantly, all three alkaloids identified as constituents of Duguetia eximia belong in this group (28). So far, duguevalline (92) is only the second oxoaporphine known to have the unusual 9,11-dioxygenation pattern. The other, oxoisocalycinine, was isolated from Guatteria discolor (50). OCH3 H3CO H3CO O N O N H3CO O O 85 O O O N O O 84 83 OCH3 N H3CO N O O N O R11O O O OCH3 86 88 : R 11 = H 89 : R 11 = CH3 87 OCH3 OCH3 OCH3 O O N O O N O O O N O H3CO O H3CO OCH3 OCH3 90 91 OCH3 92 F. Miscellaneous Aporphinoid- and Berbinoid-Related Alkaloids 1. Aminoethylphenanthrenes Four 1-aminoethylphenanthrenes, or 6,6a-secoaporphines, have been isolated from Duguetia species, these are: atherosperminine (94, from Author’s personal copy Alkaloids from the Genus Duguetia 115 D. spixiana and D. calycina), its N-oxide (95, from D. spixiana), noratherosperminine (93, from D. calycina), and methoxyatherosperminine (96, from D. spixiana). R6 N H3CO CH3 H3CO OCH3 CH3 N O CH3 H3CO H3CO H3CO H3CO 93 : R6 = H 94 :R6 = CH3 CH3 N CH3 95 96 2. Copyrine Alkaloids The relatively rare 1-azaaporphinoids are often referred to as copyrine alkaloids, by analogy with the term isoquinoline alkaloids, as copyrine is the trivial name of the 2,7-diazanaphthalene nucleus. Three 1-aza-4,5-dioxo-7-methoxy-6a,7-didehydroaporphines and two 1-aza-7-oxo-4,5,6,6a-tetradehydroaporphines were isolated from Duguetia hadrantha (16). The fact that these five unusual compounds are the only alkaloids isolated from this particular species, and that they have been found in no other Duguetia species, is probably a consequence of the antimalarial/antifungal bioassay-guided fractionation of the plant extract. They are biogenetically related to cleistopholine (104), which in this genus has only been recorded as a constituent in D. vallicola, and to other annonaceous 1-azaanthra9,10-quinone derivatives with scattered occurrence in the genera Annona, Cleistopholis, Guatteria, Meiogyne, Porcelia, Hornschuchia, and Cananga (51 56). R3 O O N N N N R6 OCH3 O R10 97 : R3 = H 98 : R3 = OCH3 99 : R6 = CH3, R10 = OCH3 100 : R6 = R10 = H 101 : R6 = CH3, R10 = H Author’s personal copy 116 Edwin G. Pérez and Bruce K. Cassels 3. Azahomoaporphines The only two azahomoaporphines found in the genus Duguetia are spiguetine (102) and spiguetidine (103), reported exclusively from a Bolivian accession of D. spixiana. They were not isolated from plant material collected in Colombia (24). They are members of a rare alkaloid structural type found only in this species, in G. sagotiana (dragabine), and in Meiogyne virgata (nordragabine), all in the family Annonaceae. O N O N CH3 R 102 : R = OCH3 103 : R = OH 4. Azaanthraquinone Cleistopholine (104), the prototype of the few natural 1-aza-9,10anthracenedione alkaloids known to date, was isolated from D. vallicola (27), and has also been found in several other Annonaceous genera. O CH3 N O 104 5. Protoberberine Styrene Adduct The structurally unique staudine (105) has only been isolated from D. staudtii (39,48). O OCH3 N H3CO OCH3 H3CO OCH3 OCH3 105 Author’s personal copy Alkaloids from the Genus Duguetia 117 IV. STRUCTURE AND CHEMISTRY A. Benzyltetrahydroisoquinolines Although the configuration of the reticuline (1) isolated from D. trunciflora was not reported, it seems likely that it is the S isomer, as in D. furfuracea, and therefore is the immediate precursor of (S)-codamine and its N-oxide (2). The small amount of 2 isolated did not allow its absolute configuration to be determined, but it is depicted here as the more likely (S)-reticuline-derived S isomer (although in the original reference it is shown with the R configuration). The berbines and the 1,2,9,10- and 1,2,10,11-dioxygenated aporphines, of which there are a few in the source plant of cis-N-oxycodamine, the Colombian accession of D. spixiana, are generally derived from (S)-reticuline (1). B. Bisbenzyltetrahydroisoquinoline The complete assignments of the 1H NMR and isochondodendrine (3) have been published (10). 13 C NMR spectra of C. Berbinoids Quite surprisingly, the presence of (R)-dicentrine (39) and its 7-hydroxy derivative duguetine (76) was reported in an unidentified Duguetia species (36). This configuration flies in the face of biogenetic theory, but seems to be supported by the negative optical rotation of both alkaloids at 589 nm, and the ORD spectrum of the latter alkaloid. Unfortunately, the only recent report on the reisolation of duguetine from Duguetia flagellaris gives no details of its identification or of its physical (including optical rotation) and spectral properties (29,30). D. Morphinandienone The stereochemistry of (9S)-sebiferine (18), which is opposite to that of the morphine alkaloids of Papaver species, was demonstrated on the basis of the crystal structure determination of its methiodide (57). Both (9S)sebiferine (18) and its enantiomer have been synthesized via p-quinol esters starting from the diastereomeric products of the lead tetraacetate oxidation of racemic N-trifluoroacetylnorcodamine in (S)-2-phenylpropionic acid (58) (Scheme 1). E. Aporphinoids The N-nitroso, non-phenolic noraporphines 51 and 52 were isolated from a 95% ethanolic extract of the leaves of D. furfuracea which was treated with 3% HCl. The authors appropriately state that N-nitrosamines can be Author’s personal copy 118 Edwin G. Pérez and Bruce K. Cassels OCH3 H3CO HO N TFA H3CO a OCH3 OCH3 H3CO H3CO H TFA N O H3CO O O + H H3CO O H O N TFA O H separation b, c, d b, c, d OCH3 OCH3 H3CO H3CO H N CH3 + H N CH3 H3CO H3CO O O Scheme 1 Reagents and conditions: a. Pb(OAc)4, (S)-2-phenylpropionic acid; b. TFA, CH3CN, 301C; c. N-deprotection; d. N-methylation. carcinogenic and/or mutagenic (59), and also remark that they ‘‘can be regarded as potential NO/NOþ donors, thus playing an important role in the regulation of many physiological functions’’ (60). However, they do not address the possibility that these N-nitroso-alkaloids are artifacts of the isolation process. Nitrates and nitrites commonly accumulate in higher plants. Their occurrence in dietary vegetables has been viewed since at least 1964 as a health hazard (61), and has been the subject of numerous subsequent Author’s personal copy Alkaloids from the Genus Duguetia 119 publications. Moreover, treatment of some secondary amine alkaloids with nitric acid has been known to lead to the formation of N-nitroso derivatives since the end of the 19th century (62), and the N-nitrosation of secondary amines occurs readily with inorganic nitrites and acid. It therefore seems possible that the N-nitrosoanonaine (51) and N-nitrosoxylopine (52) isolated by Carollo et al. were formed on acidification of the ethanol extract of the plant. What concentration of nitrate or nitrite was present in the Duguetia sample studied by these authors is a question that would seem to be worth addressing. In the opinion of the authors, nitration of isocorydine at the free C8 position, para to a phenol function to give 8-nitroisocorydine (42), should occur under very mild conditions. This reinforces the hypothesis that these unusual alkaloids are formed either in the living plant or during the extraction procedure by (presumably nonenzymatic) reaction with nitrates or nitrites present in the plant material. The 8-nitroisocorydine structure, however, does not seem to have been established unambiguously. The N-methyl 1H resonance is not reported (its 13C resonates at the normal chemical shift value of 43.9 ppm), and the mass-spectral fragmentation shows a possibly suspicious loss of NO from the molecular ion. Is it possible that this isolate is 8-nitrosoisocorydine N-oxide, with one or two apparently anomalous N-methyl resonances as described by Debourges et al. (22). It is probably important to remember that D. furfuracea is one of the three Duguetia species known to accumulate at least one aporphine N-oxide (11). A biomimetic synthesis of the unusual oxazine-condensed aporphine duguenaine (53) and some related analogs has been reported, based on the UV irradiation of an ethanol-tetrahydrofuran solution of 1-benzylidene6,7-methylenedioxy-1,2,3,4-tetrahydroisoquinoline-2-ethoxycarboxylate in the presence of iodine to produce N-ethoxycarbonyldehydro-anonaine. This was followed by N-deprotection under basic conditions and quenching with aqueous citric acid to yield the dehydroanonaine salt. The oxazine ring was introduced by treating dehydroanonaine with aqueous formaldehyde at room temperature for 24 h (Scheme 2) (63). An alternative synthesis of duguenaine (53) was published almost simultaneously, using anonaine (23) as the starting material. Anonaine was treated with N-chlorosuccinimide yielding the corresponding N-chloroanonaine. Sodium ethoxide was added to the mixture and the resulting dehydroanonaine was treated with aqueous formaldehyde under reflux for 30 min to furnish 53 (Scheme 3) (23). F. Miscellaneous Aporphinoid- and Berbinoid-Related Alkaloids Imbiline 1 (101) has been synthesized fairly recently, in seven steps, starting from 4-methoxy-1-naphthylamine, in 9% overall yield (64). Author’s personal copy 120 Edwin G. Pérez and Bruce K. Cassels O O NCO2Et O a NCO2Et O b O O N O NH O O c 53 Scheme 2 Reagents and conditions: a. UV, EtOH-THF, I2, 9.5 h; b. KOH, EtOH, reflux 18 h; c. HCHO, dioxane, rt, 24 h. O O N O H a N O 23 Cl b O O N O c O N O 53 Scheme 3 Reagents and conditions: a. NCS; b. NaOEt; c. HCHO, reflux 0.5 h. Staudine (105, relative configuration shown), isolated from D. staudtii, is a unique reverse electron demand Diels Alder adduct of jatrorrhizine (15) and 2,4,5-trimethoxystyrene, which is an abundant metabolite in this plant. Its zwitterionic, rather than phenolic, character, suggested by its high melting point (205 2061C), was revealed by the absence of any change in its UV-VIS spectrum in alkaline solution, and by the failure of Author’s personal copy Alkaloids from the Genus Duguetia 121 an attempted acetylation with acetic anhydride in pyridine in the presence of 4-dimethylaminopyridine. The presence of a C¼Nþ double bond was apparent from its IR spectrum, which exhibited a strong band at 1605 cm 1. This band disappeared on reduction of staudine (105) with sodium borohydride in methanol to afford a dihydro derivative 106 that undergoes facile reoxidation to staudine (105) in the presence of air. The 1H NMR spectrum of staudine (105) showed the presence of six methoxy groups, two single proton multiplets at d 4.45 and 5.17, and five aromatic proton singlets (one due to two protons, the others to one each). One of the methoxyl resonances (at 3.37 ppm) and one of the aromatic proton signals (at 5.32 ppm) exhibited unusual deshieldings which could be attributed to a structure with closely superimposed aromatic rings. The mass spectrum showed a weak (1%) molecular ion peak, and more abundant fragments at m/zo360. Of particular interest were three peaks at m/z 194 (90%), 179 (40%), and 151 (37%), corresponding to a trimethoxystyrene. The base peak occurred at m/z 337 (Mþ-194) with another strong signal at m/z 352 (30%, Mþ-179). These data suggested that staudine (105) contains a benzylisoquinoline moiety in addition to the trimethoxystyrene moiety, which seem to undergo a retroDiels Alder reaction in the mass spectrometer. The 13C NMR spectrum showed all the signals expected for 2,4,5-trimethoxystyrene, with the exception of the ethylene carbon resonances, and all the signals expected for corypalmine (tetrahydrojatrorrhizine, 5), except for the C14 resonance, plus additional resonances at 32.8, 34.0, and 176.3 ppm. All these data, and further tentative assignments of the sp3 13C resonances, showed that the structure of staudine (105) incorporates a 2,4,5trimethoxystyrene moiety bonded through its vinyl side chain to C8 and C13 of corypalmine, but with a C14N double bond. This was confirmed by the pyrolysis of staudine (105) under high vacuum at 1801C, which led to the sublimation of 2,4,5-trimethoxystyrene, leaving a highly polar residue. Sodium borohydride reduction of this residue afforded the previously characterized dihydrostaudine (106) and corypalmine (5) (Scheme 4). Definitive proof of the structure was provided by an X-ray crystallographic analysis, which showed unambiguously that the benzylic carbon of the styrene residue is bonded to C13 of the corypalmine moiety, and that the more distal styrene carbon atom is bonded to C8. Heating jatrorrhizine (15) and 2,4,5-trimethoxystyrene in bromobenzene at 1001C for 10 h produced only a small amount of staudine (105), identified by TLC, leading the authors to conclude that this alkaloid is not an isolation artifact (48). Nevertheless, this conclusion is still arguable considering that the same authors reported an [a]D¼0 for this alkaloid with three stereogenic carbon atoms and, as the crystal structure shows, a highly dissymmetric arrangement of the three benzene chromophores which Author’s personal copy 122 Edwin G. Pérez and Bruce K. Cassels O HO OCH3 N H3CO OCH3 N H3CO OCH3 OCH3 a b H3CO H3CO OCH3 OCH3 OCH3 OCH3 105 106 c HO d N H3CO OCH3 HO N H3CO OCH3 OCH3 15 5 OCH3 + OCH3 H3CO OCH3 Scheme 4 Conditions: a. NaBH4, MeOH; b. Air; c. 1801C, 0.01 Torr, 6 h; d. KBH4, MeOH. could be expected to result in a fairly high optical rotation. The crystal packing was not reported and it is therefore not possible to determine if the eight molecules in the unit cell have the same configuration, or if the crystal itself is racemic. It may be pointed out that 2,4,5-trimethoxystyrene, which is quite toxic to brine shrimp, but only weakly cytotoxic, has been reported as the major bioactive constituent of Duguetia panamensis Standley (no studies have been published on the alkaloids of this species) (65), and is also present in Duguetia colombiana (31). V. BIOSYNTHESIS, BIOGENESIS, AND CHEMOSYSTEMATICS No biosynthetic work has been conducted specifically on plants belonging to the Annonaceae. However, earlier studies of tetrahydrobenzylisoquinoline Author’s personal copy Alkaloids from the Genus Duguetia 123 alkaloid biosynthesis can be generalized to the more widespread Duguetia alkaloids. Regarding biogenetic speculations, some of which have been summarized in an earlier chapter of this series (51), the situation is similar. Some recent developments, both experimental and hypothetical, are reviewed here. (S)-Reticuline (1) and codamine cis-N-oxide or oxycodamine (2) lie near the base of the biosynthetic branch leading to most of the Duguetia alkaloids. As the 1,2,9,10- and 1,2,10,11-oxygenated aporphines and the berbines are all derived from (S)-reticuline (1), but not codamine, the cisN-oxycodamine of D. furfuracea can be regarded as a terminal biosynthetic product. (S)-Reticuline (1) is the biosynthetic precursor of all known berbines and the 9,10- and 10,11-dioxygenated aporphinoids, and, through the unstable 1,2-dehydroreticuline, is also the precursor of (R)-reticuline, the common precursor of most morphinandienone alkaloids. Reasoning biogenetically, ( )-dicentrine (39) should originate by direct C8 C6u coupling of (R)-reticuline. It is therefore of interest to note that 1,2dehydroreticuline synthase, the enzyme at the branching point that separates (R)- and (S)-reticuline metabolites, has been partially purified and shown to not require a redox cofactor, accepting both (S)-reticuline and (S)-norreticuline as substrates (66). The occurrence of isochondodendrine (3) as the sole Duguetia bisbenzyltetrahydroisoquinoline parallels the limited occurrence of benzyltetrahydroisoquinoline dimers in Guatteria. In the largest genus in the Annonaceae, these alkaloids, although many in number, appear to be restricted to G. boliviana, G. guianensis, and G. megalophylla (51,67). Guatteria gaumeri, reported to contain a bisbenzylisoquinoline, is a misnomer for Malmea gaumeri, now viewed as a synonym of Malmea depressa (68). Moreover, cladistic analysis indicates that the split between the branches leading to Malmea (the short branch clade of the Annonaceae) and to Duguetia and Guatteria (the long-branch clade) must have occurred about 60 million years ago, 20 million years before the differentiation of the latter genera (46). Within the long-branch clade, the only other genera for which bisbenzylisoquinolines have been recorded are Isolona, Uvaria, and Xylopia. This suggests that the cytochrome P450 oxidases that presumably catalyze the intermolecular oxidative phenol couplings (two in succession in the case of isochondodendrine) of two coclaurine units (69) are poorly expressed in this group. In the last few years, particularly important contributions have been made to the knowledge of the berberine bridge enzyme. This protein, incorporating a unique, bi-covalently attached FAD prosthetic group (70), catalyzes the conversion of (S)-reticuline to (S)-scoulerine by oxidation of the N-methyl group and coupling ortho to the phenol group of the benzyl ring (71,72). A mechanism has been proposed involving the Author’s personal copy 124 Edwin G. Pérez and Bruce K. Cassels removal of hydride from the N-methyl group by the FAD cofactor, and concerted carbon carbon coupling combined with base-catalyzed proton abstraction (73). The enzyme also oxidizes the berbine alkaloid scoulerine to the protoberberine dehydroscoulerine, resembling (S)tetrahydroprotoberberine oxidase (STOX) and canadine oxidase in this regard (74). (S)-Tetrahydroprotoberberine oxidase converts (S)-tetrahydrocolumbamine to columbamine in the metabolic pathway leading to berberine, jatrorrhizine, and palmatine in Berberis species (75). Canadine oxidase catalyzes an alternative route in which formation of the dioxole ring precedes the dehydrogenation leading to berberine (76). (S)-Reticuline (1) is not the exclusive berbine precursor. Berberine bridge enzyme of Eschscholtzia californica, heterologously expressed in insect cells, transforms other (S)- (but not R-configured) tetrahydrobenzylisoquinolines with a 2u-hydroxy group into (S)-berbines, apparently regardless of the substitution pattern on the benzene ring of the isoquinoline moiety of the precursor (77). In Corydalis and Macleaya cell cultures both (S)-reticuline and (S)-protosinomenine (the isomer of reticuline with the positions of the ring A hydroxy and methoxy groups interchanged), but not their enantiomers, undergo the analogous cyclization to (S)-scoulerine and tetrahydropalmatrubine (its methoxy derivative at C2) (78). On the other hand, when racemic laudanine (the 7O-methyl ether of reticuline) was fed to the cells, both enantiomers of scoulerine and of the 10,11-dioxygenated berbine corytenchine were formed, in different enantiomeric ratios (78). N-Methyltetrahydropalmatine (8) and the analogous N-methylstylopine and N-methylcanadine are synthesized in opium poppy from the corresponding racemic berbines by a recently cloned and characterized S-adenosyl-L-methionine:tetrahydroprotoberberine cis-N-methyltransferase (TNMT) which, however, does not modify (S)-scoulerine (79). The stereochemistry of the products was not determined. TNMT activity was detected in several other members of the Papaveraceae, but not in representatives of the Berberidaceae, Menispermaceae, and Ranunculaceae. It remains to be seen if this, or some similar, enzyme is active in the Annonaceae, and specifically in D. furfuracea. It is worth pointing out that no 2-hydroxyberbines or protoberberines have been found in Duguetia, although there are a number of occurrences of 3-hydroxyberbines [discretamine (4), corypalmine (5), and discretine (10)] and the oxidation products of 5 and 10 [jatrorrhizine (15) and dehydrodiscretine (16)]. Assuming that all berbines are formed from (S)norreticuline by a berberine bridge enzyme (73,77,78), this would seem to imply that the formal translocation of a methyl group from the methoxyl at C2 to the C3 hydroxyl group is a practically universal occurrence in this genus. In the rather well-studied genus Guatteria, coreximine (2,11dihydroxy-3,10-dimethoxyberbine, one of the putative precursors of the Author’s personal copy 125 Alkaloids from the Genus Duguetia whole series) is present in two out of four berbine-accumulating species reviewed two decades ago (51). The fact that only 4 out of 18 Guatteria species were shown to contain berbines (and protoberberines were not recorded) suggests that the berberine bridge pathway is considerably less active in Guatteria than in Duguetia. The presence of spiduxine (13) and thaicanine (14) in Duguetia is another indication of the greater ability of this genus to elaborate the berbine skeleton. Regarding the (tetrahydro)retroprotoberberine spiduxine (13) (21), Shamma proposed in his 1972 treatise on the isoquinoline alkaloids that the related mecambridine, orientalidine, and their oxidation products PO-5 and PO-4 might arise from a berbine by cleavage of the N C8 bond giving a 1-benzyl-3,4-dihydroisoquinoline that could be reduced to its tetrahydro counterpart, N-methylated, and a new ‘‘berberine bridge’’ built (80). This scheme is illustrated for the case of spiduxine (13) (Scheme 5). Elegant though this model may appear to be, it lacks experimental support. Considering the ability of Duguetia species to introduce onecarbon units in the unexpected C7 position (viz. 53 55), for the sake of parsimony one can also speculate that spiduxine is generated by formylation ortho to the phenolic hydroxyl of 2-O-methylcoreximine. Nevertheless, a few years ago the unusual structure of a new benzyltetrahydroisoquinoline alkaloid named (þ)-argenaxine (106) (isolated from Argemone mexicana, Papaveraceae) was published (81), with a H3CO H3CO N H3CO N H3CO OH OCH3 OCH3 O H3CO OH H3CO N H3CO N H3CO CH3 OCH3 13 Scheme 5 O OH Proposed biogenesis of spiduxine. OCH3 O OH Author’s personal copy 126 Edwin G. Pérez and Bruce K. Cassels regio- and stereochemistry compatible with its hypothetical formation by cleavage of an (S)-berbine and the possibility of it being a precursor of a tetrahydroretroprotoberberine (or retroberbine). O NH O H 106 OCH3 OH OCH3 Interestingly, none of the berbines or protoberberines isolated from Duguetia have a methylenedioxy group, suggesting that the enzyme that effects closure of this ring in the many Duguetia methylenedioxyaporphinoids, supposedly a member of the CYP719A subfamily of cytochrome P450s (82), does not accept the geometrically extended berbine skeleton. The apparently unusual stereochemistry of the morphinandienone (9S)-sebiferine (18) seems to be justified by the fact that, at least in Cocculus laurifolius (Menispermaceae), the biosynthetic conversion of (S)and (R)-reticuline (1) into sebiferine (18) is not stereospecific (83). The C C phenolic coupling reaction of (R)-reticuline (1) to salutaridine is the first morphinandienone-forming step, at least in morphine biosynthesis (84), but the enzyme that catalyzes this reaction has not yet been characterized. Aporphines are believed to be formed by C C phenolic coupling between C8 and C2u C6u of a benzylisoquinoline or, via an intermediate proaporphine, between C8 and C1u. An enzyme catalyzing the first route, CYP80G2, has now been cloned and characterized from Coptis japonica (85). This enzyme converts (S)-reticuline (1) to its direct coupling product (S)-corytuberine. If an analogous enzyme is operating in Duguetia, it should be responsible for the formation of isocorydine (41), an O-methylation product of corytuberine and the probably derived norisocorydine (40) of D. vallicola and D. furfuracea. The presence of the proaporphine glaziovine (19) in D. vallicola is somewhat surprising considering that its likely biogenetic derivatives, (R)-aporphines with the 1-hydroxy-2-methoxy, or the 1,10-dihydroxy-2-methoxy, or 1-hydroxy2,10-dimethoxy substitution patterns seem to be completely absent from the genus. The 9,11-substitution pattern in the D ring of aporphines is of taxonomic significance in the Annonaceae, as already noted by Roblot et al. in 1983 (20). Only one aporphine with this structural feature has been reported in the Ranunculaceae and Menispermaceae (86,87) and these Author’s personal copy Alkaloids from the Genus Duguetia 127 alkaloids are mainly present in Guatteria and Duguetia (17,20,21,27, 29,30,50,88). In the review on Guatteria alkaloids published in this series (51), it was proposed that one of the ring D substituents might be introduced meta to the other, once the aporphine skeleton had been generated from the appropriate proaporphine, stating that either the C11-oxygenated puterine (31 in this review) or guadiscine (7,7-dimethyl9-methoxy-1,2-methylenedioxy-6,6a-didehydronoraporphine) could be precursors of the 9,11-dioxygenated alkaloids, and that the process might not be very regiospecific. Actually, guadiscine (present in G. discolor and G. melosma) is only a reasonable precursor of guadiscoline (7,7-dimethyl-9,11-dimethoxy-1,2-methylenedioxy-6,6a-didehydronoraporphine, only found in G. discolor), while 31 would be a possible precursor of isocalycinine, discoguattine, oxoisocalycinine, guacolidine, and guacoline, all of which are Guatteria alkaloids, and are not isolated from the genus Duguetia. It is intriguing to note that the only American Duguetia species known to accumulate a 7-methoxyaporphinoid [pachypodanthine (78), in the abundant Amazonian D. flagellaris] should grow down to the coast of the Brazilian states of Pará and Maranhão. This part of the South American Gondwana fragment lies opposite to the western reaches of the Gulf of Guinea and Sierra Leone, to which it was formerly attached, and where D. staudtii now grows. In the recent analysis of the anatomical and morphological data of Duguetia and closely related genera (6), D. confinis and D. staudtii, earlier described as Pachypodanthium, are placed close to the African species Duguetia barteri (Benth.) Chatrou (also formerly Pachypodanthium) and Duguetia dilabens Chatrou et Repetur (a new species) and to D. riberensis of Venezuela, and presumably Colombia. It would be most interesting if the latter plant could be collected and analyzed to determine if it contains 7-methoxylated aporphinoids, like the reasonably well-studied D. confinis and D. staudtii. Pachystaudine (82) and norpachystaudine (81) are said, on the basis of their CD spectra, to have the 6aS configuration. This stereochemistry is exceptional for aporphinoids devoid of substituents on ring D, which are generally believed to arise through the dienol benzene rearrangement of proaporphines derived from (R)-coclaurine or norcoclaurine. This apparent anomaly parallels the identification of the (R)-9,10-dioxygenated ( )-dicentrine (39), from the leaves of an unidentified Amazonian species (36). It was argued convincingly on the basis of their common 6aR configuration (20), that the N-formylnoraporphines, found for the first time in Duguetia species, cannot be metabolites of N-formyl-1-benzyltetrahydroisoquinolines originating from the cleavage of ring C of (14S)berbines as suggested earlier (89). In addition, it was indicated that the Author’s personal copy 128 Edwin G. Pérez and Bruce K. Cassels simultaneous presence of N-formyl-, N-methyl-, and noraporphines, and the accumulation of the latter as major alkaloidal constituents in D. calycina and D. obovata, pointed to the noraporphines as final biogenetic products (20). Although the precise sequence was not suggested, analogy with the catabolism of N-methyl groups in animals allows the sequence aporphine N-formylnoraporphine noraporphine to be proposed. Noraporphines are therefore likely precursors of the 7- and 4-hydroxynoraporphines, 7-oxo-, and 4,5-dioxoaporphines, and finally the 1-azaaporphinoids (copyrine alkaloids), aristolactams, azaanthraquinones, and their putative derivatives. The isolated occurrence of duguevalline (92) in D. vallicola (27) and oxoisocalycinine in G. discolor (50) as the only oxoaporphines with the 9,11-dioxygenation pattern is insufficient to suggest any chemosystematic trend. On the other hand, it might be significant that Colombian D. spixiana accumulates seven N-oxides (five of them aporphine N-oxides), while only one each are found in D. furfuracea, D. flagellaris, and Bolivian D. spixiana, and only two in a single Guatteria species (G. sagotiana) (51). Aminoethylphenanthrenes or secoaporphines are thought to arise by the Hofmann elimination of quaternary aporphine alkaloids (the quaternization and elimination products are commonly termed ‘‘methines’’), and this indeed would seem to be the case for atherosperminine (94, nuciferine methine) and methoxyatherosperminine (96, 3-methoxynuciferine methine). The formation of atherosperminine N-oxide (95) appears to follow an important catabolic trend for Colombian D. spixiana. Noratherosperminine (93) would presumably arise through the N-demethylation of atherosperminine (94), probably catalyzed by a cytochrome P450. An alternative explanation would involve an anomalous Hofmann elimination reaction of the tertiary nuciferine (necessarily in its N-protonated form?). Although such a reaction has been documented in vitro for boldine (90) in refluxing ammonium acetate solution, it seems extremely unlikely that it should occur nonenzymatically in vivo. Therefore, one would have to assume the existence of a ‘‘Hofmannase’’ for which there does not seem to be any precedent. It is interesting that only nuciferine and 3-methoxynuciferine are involved in the biogenesis of these aminoethylphenanthrenes. Nornuciferine (22) and 3-hydroxynornuciferine (25) have been shown to accumulate only in Bolivian D. spixiana, and the former also in D. flagellaris, but their tertiary and quaternary analogs, the expected precursors of their ring-opened products, have not been recorded for any Duguetia species. This seems remarkable in view of the presence of the close nuciferine congener anonaine (23) in Bolivian D. spixiana (and also D. furfuracea), but not its N-methyl homolog roemerine, its quaternary Author’s personal copy 129 Alkaloids from the Genus Duguetia derivative, or its seco counterpart. In all, 26 aporphines sensu stricto, including several nor- and two quaternary aporphines, are listed above, and only two of them can be envisioned as precursors of the Duguetia aminoethylphenanthrenes. On the other hand, the quaternary N-methylglaucine (36, from D. furfuracea) and N-methylguatterine (66, from D. odorata) do not seem to undergo ring opening in this genus. The phytochemical literature records a large number of aminoethylphenanthrenes, many from different Annonaceous genera, apparently derived from aporphines with most of the various substitution patterns. Therefore, the very limited occurrence of these alkaloids in Duguetia suggests the hypothesis that they are the products of a metabolic route involving a highly specific enzyme at some key step, possibly the ‘‘Hofmannase’’ mentioned above. The copyrine alkaloids or 1-azaaporphinoids can be viewed as aporphine derivatives in which ring A has been opened (e.g., by extradiol cleavage of a 1,2-catecholic aporphine between C1 and C11b) with subsequent reclosure through condensation with an ammonia molecule (91). Taylor’s biogenetic proposal deriving the azafluoranthene, diazafluoranthene, tropoloisoquinoline, 1-azaanthracene, and azafluorenone alkaloids from 1,2-dihydroxy-7-oxoaporphine (liriodendronine) through an initial ring A cleavage (92,93) has been extended to explain the formation of the hadranthines and imbilines via formal 1,4-hydrogenation of the ketoimine function and stabilization by O-methylation, either preceded, or followed by, conversion of pyridine ring B to the b-ketolactam function (16). An alternative pathway to the 7-methoxylated 1-aza-4,5-dioxoaporphinoids or the 4,5-dioxocopyrines of D. hadrantha, not requiring a reduction step, might start from N-methylliriodendronine, in which the C7 oxygen function is already a phenoxy group, particularly in view of the presence of many 7-hydroxy- and two 4-hydroxy-7methoxyaporphines in Duguetia (Scheme 6). The proposal for the late oxygenation of C4 and C5 could be circumvented by a parallel route to the 4,5-dioxocopyrines starting from 1,2-dihydroxy-4,5-dioxoaporphine, which leaves open the possibility of a monooxygenase-catalysed hydroxylation at C7 (Scheme 7). HO O HO HO + NCH3 O- O COOH + NCH3 O NCH3 OCH3 OCH3 Scheme 6 Initial steps of a proposed biogenetic pathway to 4,5-dioxocopyrines starting from N-methylliriodendronine. Author’s personal copy 130 Edwin G. Pérez and Bruce K. Cassels HO O HO HO O NH COOH O O O NH Scheme 7 First step of a proposed biogenetic pathway to 4,5-dioxocopyrines starting from 1,2-dihydroxy-4,5-dioxoaporphine. A biogenetic proposal to account for the formation of azahomoaporphines was published 20 years ago in this series (51). According to that hypothesis, spiguetine (102) and spiguetidine (103) of the Bolivian sample of D. spixiana might be derived from the 7-hydroxyaporphines oliveridine (68) and roemerolidine (69), which are the major alkaloids of the same plant. It was suggested that a-aroylpyridine derivatives, and more specifically 1-azaanthracene-9,10-diones, such as cleistopholine (104), might undergo decarbonylation catalyzed by a metalloenzyme (93). This has now received indirect support from the formation of metal complexes of liriodenine (83) which confirm the metallophilicity of the 7-oxoaporphine arrangement of a pyridine nitrogen and a carbonyl oxygen and, presumably, of related systems (94). Some striking resemblances in the alkaloid chemistry of Duguetia and Guatteria were pointed out by Cavé in 1984, as indicating the possible proximity of these genera (95). At that time, it was known that both Duguetia and Guatteria species accumulate 7-alkylated aporphinoids and N-formylnoraporphines. It was then suggested that the unusual oxazine-condensed aporphine system of duguenaine (53) and duguecalyne (54) might arise from ring closure of N-formyl-7methylaporphinoids or, alternatively, their 7-formyl-N-methyl counterparts, indicating that such potential intermediates had already been found in D. spixiana (duguespixine, 55) and Guatteria trichostemon (trichoguattine, the 1,2-methylenedioxy analog of 55). In fact, the related 9-hydroxylated belemine and goudotianine have also been isolated from a couple of Guatteria species (96,97). Another common feature pointed out by Cavé was the 9,11-dioxygenation pattern of some Duguetia and Guatteria aporphinoids. At that time (1984), he noted that the phenol function is located at C9 in Guatteria and at C11 in Duguetia. This is not strictly so, as discoguattine, guacoline and guadiscoline are 9,11-dimethoxylated aporphinoids, but the first two alkaloids could obviously be formed by O-methylation of their putative 9-hydroxy precursors isocalycinine and guacolidine. Author’s personal copy Alkaloids from the Genus Duguetia 131 It is worth mentioning that D. calycina and D. spixiana, the only Duguetia species known to contain 1-aminoethylphenanthrenes, are classed in the section Sphaerantha, and thus the occurrence of this small group of alkaloids might be of chemosystematic significance. Interestingly, atherosperminine, N-oxyatherosperminine, noratherosperminine, together with the 2-O-demethylated atherosperminine analog argentinine (N-methylasimilobine methine), are the only 6,6a-secoaporphines isolated from the larger Annonaceous genus Guatteria, and that from the single species G. discolor (50,98). However, G. discolor appears to have arisen from fairly recent (Pliocene or Pleistocene) diversification events within Guatteria (99), placing it at a considerable evolutionary distance from the Eocene split that presumably originated Duguetia (46), and suggesting that aminoethylphenanthrene accumulation is not an ancestral character, but rather one that has appeared in a scattered fashion in plants that synthesize aporphines, either by convergent evolution or by cross-colonization by endophytic fungi with the relevant synthetic abilities. As in Duguetia, the Guatteria aminoethylphenanthrenes are formally and exclusively derived from ring D-unsubstituted aporphines. As in the case of the copyrine alkaloids, it has been proposed that the azahomoaporphine skeleton arises by oxidative cleavage of the aporphine system, in this case between C6a and C7, and reclosure incorporating an ammonia molecule (100). Finally, if staudine (105) is in fact an enzymatic product, one would have to invoke catalysis by a Diels Alderase to explain its formation. A striking aspect of the known alkaloid chemistry of Duguetia is the apparent lack of correlation between the structures of the isolated alkaloids and the morphologically based classification of the genus into sections. Although the large section Duguetia, for example, seems to be well-supported on morphological and genomic grounds, none of the (relatively few) individual alkaloids isolated from D. odorata and D. stelechantha have been found in the seemingly exhaustive studies of D. furfuracea, classed in the same section. One would like to find a more convincing degree of chemosystematic order in such an extensively studied genus, but this will probably be impossible without more exhaustive studies of some species, and adequate quantification of the individual alkaloids in crude extracts rather than the isolated yields, probably using a metabolomic (or metabonomic, or metabolic profiling) approach (101,102). With a significantly more complete picture, it should become possible to reasonably address the fascinating question of how the diverging biosynthetic pathways present in Duguetia are regulated. Author’s personal copy 132 Edwin G. Pérez and Bruce K. Cassels VI. ETHNOPHARMACOLOGY AND PHARMACOLOGY Surprisingly little has been published on the ethnopharmacology of Duguetia species, as recognized by the authors of one of the most recent papers discussed here (13). A possible explanation is that most of these plants grow in the Amazon region and, if they have medicinal or related uses, are only employed by ethnic groups whose practices have been poorly recorded by outsiders. As is the case with the bulk of ethnopharmacological data, traditional uses are frequently difficult or impossible to ascribe to medical conditions recognized by Western science, and even less so to pharmacological mechanisms. Moreover, in the vast majority of instances, the effectiveness of these practices has not been substantiated scientifically through direct observation. Furthermore, the literature reveals an unfortunate tendency to ascribe a biological activity of a plant or a plant extract, obtained with little regard to the traditional mode of preparation, to whatever can be isolated (and often, but not always, biologically evaluated). Finally, there is an almost complete absence of the quantitative analysis of the active constituents, which can lead to the erroneous conclusion that a substance present in insufficient amounts to produce any effect is responsible in the field for test results obtained with the pure compound. D. furfuracea has two recorded uses in traditional medicine: its powdered seeds are mixed with water and used to kill lice, and an infusion of the twigs and leaves is used against rheumatism (13). D. flagellaris is also used to treat rheumatism as an infusion in sugar cane spirit (30,103). Duguetia glabriuscula is said to be used to kill cockroaches, although the report does not mention what part of the plant is insecticidal (104). The insecticidal uses of Duguetia species are probably not related to their alkaloid content, but rather to the presence of the socalled ‘‘Annonaceous acetogenins,’’ characteristic of many Annonaceae, but not yet reported for the genus Duguetia. It is worth noting that the use of powdered Annonaceae seeds as insecticides was first recorded four centuries ago (105). D. confinis is used in tropical Africa as a cough suppressant and analgesic, particularly for toothache (37). The stem bark of D. staudtii is used by some populations in the Ivory Coast as an arrow poison ingredient. The bark is also frequently used in traditional medicine for several indications: ground to a pulp with kola nut it is used to treat gastrointestinal pain and locally, mixed with Ficus exasperata leaves, as an anti-inflammatory; it is also considered an analgesic, and some populations in the Congo use it for cough, and for difficulty in breathing. The Pomo tribe, also in the Congo, claims that the bark of this species is a purgative and an aphrodisiac (39). Author’s personal copy Alkaloids from the Genus Duguetia 133 No ethnopharmacological data seem to have been published for any other Duguetia species. In contrast, although information is lacking regarding the pharmacology of most individual Duguetia alkaloids, the last two decades have seen an extraordinary number of papers on the biological properties of a few alkaloids that are either abundant, characteristic, or recognized as active principles of other plants, and are also present in Duguetia. Additionally, some generalizations can be made safely as to the related pharmacological activities of substances that are close structural congeners. A. Benzyltetrahydroisoquinolines (S)(þ)-Reticuline (1) is a dopamine receptor antagonist, blocking the actions of the dopamine agonist apomorphine, causing decreased locomotor activity and producing catalepsy in rats (106,107). These effects seem to be elicited by the blockade of postsynaptic striatal dopamine receptors (108). Dopaminergic antagonism by reticuline (1) appears to be rather weak, however, and has not attracted much interest, although it might be involved in the central depressant effects observed in rats and mice (109). Reticuline (1) inhibits dopamine uptake and at high concentrations is toxic to dopaminergic and GABAergic neurons. It has therefore been suggested that it might be involved in the genesis of the atypical Parkinsonism of the French West Indies, associated with the consumption of fruit and infusions of the reticuline-containing Annona muricata (110). (S)(þ)-Reticuline (1) is also a weak neuromuscular (nicotinic cholinergic) blocker (111). In addition, it reduces the contractile force of guinea pig heart by blocking calcium channels (112). (S) (þ)-Reticuline-induced uterine relaxation and vasorelaxation by L-type Ca2þ channel blockade have also been demonstrated (113,114). Nevertheless, the cardiovascular effects of reticuline (1) appear to depend on the blockade of Ca2þ entry and on the inhibition of Ca2þ release from norepinephrine-sensitive intracellular stores, and by cholinergic (muscarinic) stimulation and nitric oxide synthase activation in the vascular endothelium (115). (S)(þ)-Reticuline (1) has antiplatelet aggregation activity (116). It shows some antifungal activity (117), and is rather weakly antiplasmodial (118). It is also claimed to accelerate hair growth (119). Reticuline, at 20 mg/kg, administered intraperitoneally, is significantly antinociceptive in the acetic acid-induced mouse writhing test, and quenches diphenylpicrylhydrazyl (DPPH) radicals with a scavenging concentration (SC50) of 47 mg/mL (143 mM) (120). The latter antioxidant property could well be related to its effects on inflammation and pain. Author’s personal copy 134 Edwin G. Pérez and Bruce K. Cassels Nothing is known about the pharmacology of N-oxycodamine (2) or, in fact, of other benzyltetrahydroisoquinoline N-oxides. B. Bisbenzylisoquinoline Isochondodendrine (3) was mentioned more than 50 years ago as a possible agent for the treatment for dysmenorrhea, but this lead does not seem to have been pursued (121,122). The only recent work found refers to the potent antiplasmodial activity of isochondodendrine (3) in vitro (IC50¼0.10 mg/mL) (123,124), which makes one wonder if D. furfuracea might be used to treat fever or, more specifically, malaria, in the area where it grows. C. Berbinoids Discretamine (4) is a potent a1-adrenergic blocker, comparable in potency and basic pharmacology to the hypotensive drug phentolamine. It also blocks a2-adrenoceptors and 5-HT2 serotonin receptors, at several times higher concentrations, and seems to be devoid of action at acetylcholine, histamine, leukotriene, thromboxane, prostaglandin F2a, or angiotensin II receptors (125). Its action on a1-adrenoceptor subtypes is selective for a1D over a1A and a1B (126). Discretamine (4) antagonizes the contraction of human hyperplastic prostate tissue elicited by phenylephrine, electrical stimulation, or high Ca2þ (127). Its antiplatelet aggregation effect is another potential beneficial action of this alkaloid (128). Discretamine (4) is hypotensive in rats at doses between 0.01 and 10 mg/kg. A series of in vitro experiments suggests that the hypotensive effect of discretamine (4) is probably due to peripheral vasodilation related to nitric oxide release from the vascular endothelium (34). Of all the berbine alkaloids recorded as Duguetia constituents, THP (7) is by far the most studied in relation to its pharmacology, probably because its (S)( )-enantiomer (rotundine) and the racemic mixture are active constituents of the Asian drugs Stephania rotunda and Corydalis racemosa, respectively. As far back as 1970 (S)-THP, with the generic name ‘‘gindarin,’’ was evaluated for dermatological use in the treatment of neurodermatitis and alopecia areata, but this study does not seem to have progressed any further (129). (7)-THP (7) is listed in the Chinese Pharmacopoeia as an analgesic with sedative hypnotic effects. This alkaloid, together with its close analogs tetrahydroberberine and tetrahydrocoptisine, though apparently not tetrahydrojatrorrhizine (5), were shown to exhibit central depressant effects in mice and rats similar to those of the well-known neuroleptic chlorpromazine, leading to the suggestion that these berbines might represent ‘‘a new type of tranquilizer’’ (130). (7)-THP (7) was later Author’s personal copy Alkaloids from the Genus Duguetia 135 recognized as a dopamine, and, to a lesser extent, noradrenaline and serotonin depletor with an action similar to reserpine (131). In the former Soviet Union, the S-enantiomer, ‘‘gindarin,’’ was subjected to a preclinical study (in rats) in the framework of its possible use as a tranquilizer (or neuroleptic), and was found to be embryotoxic (132). (S)-THP (7) was subsequently shown to be a dopamine antagonist, while the R-isomer appears to be responsible for dopamine depletion (133,134), acting on both pre- and postsynaptic receptors (135). These dopaminergic actions probably explain the neuroleptic-like activity of both (S)- and (7)-THP (7). Radioligand displacement studies showed that (S)( )-THP (7), but not its enantiomer, has affinity for D2(-like) receptors (136). Subsequently, in vivo data were acquired showing that this alkaloid lacks agonistic effects (137). It has been shown recently that (S)( )-THP (7) binds with high affinity (Ki¼94 nM) to rat D1 dopamine receptors, while a 3:1 mixture, in which the R-enantiomer predominates, has only micromolar affinity (138). (7)-THP (7) decreases motor activity in rats, producing rigidity (or catalepsy?) at higher doses, apparently due to enhanced turnover of dopamine, although increased turnover is also observed for norepinephrine and, at higher doses, for serotonin (139). The antinociceptive action of (S)( )-THP (7) is attributed to its D2 antagonism in the striatum and nucleus accumbens, thus enhancing the activity of the brainstem descending pain modulation system (140 142). This effect might be reinforced by endogenous opioid release, as chronic administration of the alkaloid increased the Leu-enkephalin content in the rat striatum (143), and lesion of a predominantly b-endorphin pathway abolished the analgesic action of (S)-THP (7) (144). The hypotensive and heart rateslowing effects of (7)-THP (7) have also been related to D2 antagonism (145). Nevertheless, other mechanisms are clearly at work in the cardiovascular actions of this alkaloid, whether the S isomer or the racemic mixture. Calcium channel blockade and a1 and a2 adrenoceptor antagonism were first implicated in 1989 (146). (S)-THP (7) is also a subtype nonselective a-adrenoceptor antagonist (147). Experiments in rats demonstrated the protective effects of the S-enantiomer in experimental myocardial infarction, apparently related to its action on calcium channels (148,149). The first clinical results showing the effectiveness of (S)( )-THP in patients with atrial fibrillation or paroxysmal tachyarrhythmias were published in 1993 (150,151). (7)-THP (7) is used for the treatment of pain, but reports have surfaced of severe cardiac and neurological toxic effects from abuse of this drug, and it has been suggested that these problems are also due to calcium channel blockade (152). Although the peripheral effects on calcium channels and adrenergic receptors are supported by later studies, there are strong indications that the cardiovascular effects of (7)-THP (7) are due, at least Author’s personal copy 136 Edwin G. Pérez and Bruce K. Cassels in part, to hypothalamic dopamine antagonism and/or 5-HT2 serotonergic agonism (153,154). The racemic mixture also induces hypothermia, which is attenuated by brain serotonin depletion or 5-HT2 serotonergic receptor activation, again indicating a central serotonin antagonist action of the drug (155). Pretreatment with (7)-THP (7) suppresses behavioral activation by picrotoxin (a noncompetitive GABAA receptor inhibitor) in rats, suggesting that this alkaloid might suppress epileptic seizures through inhibition of dopamine release (156). In this connection, the alkaloid was tested on the development of seizures in animals with electrically kindled amygdala, and found to be very effective as an antiepileptogenic and anticonvulsant agent in this model (157). It was subsequently shown that THP (7) is a positive allosteric modulator of GABAA receptors, thus sharing some of the pharmacological properties of the antiepileptic barbiturates and benzodiazepines (158). An independent study showed that orally administered (7)-THP (7) exhibits anxiolytic-like actions in mice, and that these effects are abolished by coadministration of a benzodiazepine antagonist, suggesting that THP interacts with the benzodiazepine site of the GABAA receptor (159). In rats, (S)( )-THP (7) inhibits methamphetamine- and cocaineinduced conditioned place preference, a preliminary test of possible antiaddictive activity in humans (160,161). Furthermore, it reduces cocaine self-administration and reinstatement, suggesting that it could also be useful in the treatment of cocaine addiction (162,163). Studies in rodents and in humans suggest that (S)( )-THP (7) can ameliorate opioid drug craving and increase abstinence (164 165). THP (7) is a weak inhibitor of the mitochondrial respiratory chain (166), and binds poorly to DNA (dissociation constants of the order of 10 4 M, with the R-enantiomer binding about twice as strongly as the Senantiomer) (167). In line with these results, THP (7) and also xylopinine (12) are only weakly cytotoxic (168). Other miscellaneous effects of THP (7) have been examined in relatively little detail. The racemic alkaloid produces significant decreases in thyroid function in hyperthyroid rats, apparently by inhibiting the release of thyrotropin-stimulating hormone (169). (7)-THP (7) attenuates several parameters related to neuronal damage caused by heatstroke in rats (170). (S)-THP (7) has several beneficial actions during acute cerebral ischemia-reperfusion in rats (171 174), and depresses the expression of adhesion molecules induced by lipopolysaccharides, suggesting that it might be useful in the treatment of inflammation (175). In this connection, and considering that free radicals are involved in inflammation, it should be pointed out that THP (7) exhibits antioxidative activity of similar potency to phenolic flavonoids in the lipid peroxidation and hemolysis assays (176). The racemic alkaloid Author’s personal copy Alkaloids from the Genus Duguetia 137 protects against carbon tetrachloride-induced liver damage in mice, which is also related to the formation of free radicals (177). THP (7) causes paralysis in the domestic fowl parasitic worm Raillietina echinobothrida at 1, 2, and 5 mg/mL, apparently related to disturbance of the nitric oxide signaling pathway (178). The antiplasmodial activity of thaicanine (14) was demonstrated almost two decades ago, at low-to-submicromolar concentrations, against the chloroquine-sensitive Plasmodium falciparum D-2 strain and the resistant W-2 strain (120). Discretine (10) inhibits the growth of P. falciparum (chloroquine-resistant FcB1/Colombia strain) with IC50¼ 1.6 mM, and is practically noncytotoxic against KB cells (179). THP (7) and xylopinine (12) are only weakly active against P. falciparum (IC50¼32 and 52 mM, respectively) (180). D. Protoberberines Jatrorrhizine (15), only isolated to date, in the Annonaceae, from D. trunciflora, is mentioned in a large number of pharmacological papers. Jatrorrhizine (15) lowers arterial blood pressure in normotensive dogs (181). It blocks a1 and a2 adrenergic receptors with moderate potency and exhibits some antihypertensive and heart rate-slowing activity in rats, although at higher concentrations these effects are reversed (182). Jatrorrhizine (15) inhibits both monoamine oxidase isoforms (MAO-A and MAO-B) of rat brain with IC50 values of 4 and 62 mM, respectively (183). It also inhibits rabbit platelet aggregation in vitro (184), and acetylcholinesterase inhibition by jatrorrhizine (15) has also been reported (185). Antimicrobial activity of jatrorrhizine (15) against Mycobacterium smegmatis was demonstrated at concentrations of less than 100 mg/mL (184). It was recently tested against a panel of human dermatophytes and yeast-like fungi, exhibiting minimal inhibitory concentrations (MIC) between 62.5 and 125 mg/mL against Epidermophyton, Trichophyton, and Microsporum species, and 250 and 500 mg/mL against Candida tropicalis and Candida albicans, respectively; all better results than those obtained with berberine. However, it was inactive against Scopulariopsis brevicaulis (186). Bifonazole and fluconazole were used as positive controls, the former exhibiting MIC values above 100 mg/mL for all strains, but Epidermophyton floccosum, and the latter also, with the additional exception of Trichophyton rubrum. Tests against 20 strains of Staphylococcus (including 14 of S. epidermidis) and 20 strains of Propionibacterium acnes, and 20 Candida strains (including 17 of C. albicans) showed that the antibacterial potency of jatrorrhizine (15) is less than that of berberine, and that both alkaloids are inferior to commonly used antibacterial drugs. However, jatrorrhizine (15) may be a good lead for the Author’s personal copy 138 Edwin G. Pérez and Bruce K. Cassels development of more effective antifungal agents than those in current use (187). This alkaloid (15 is active) in vitro against two different clones of P. falciparum with IC50 values of 0.422 and 1.607 mg/mL, potencies comparable to that of quinine, however, in an in vivo (mouse) screen against Plasmodium berghei it was inactive (188). Against the P. falciparum multidrug-resistant strain K1, it exhibited IC50¼3.15 mM, (corresponding to 1.1 mg/mL), and showed very modest activity against Entamoeba histolytica (189). In cultures of Babesia gibsoni, an important parasite in dogs and a member of a genus causing babesiosis in other carnivores, ruminants, and horses, jatrorrhizine (15) inhibited growth at low-tomoderate concentrations (190). Dehydrodiscretine (16) inhibits the growth of P. falciparum with IC50¼0.64 mM (multidrug-resistant K1 strain) (189), and 0.9 mM (chloroquine-resistant FcB1/Colombia strain) (179). Jatrorrhizine (15) and dehydrodiscretine (16) have negligible cytotoxicity against KB cells (179,189). The interaction of jatrorrhizine (15) with DNA resembles that of ethidium bromide, the classical DNA intercalator (191). Binding to calf thymus DNA reveals two different binding sites with dissociation constants of about 25 and 35 mM (192). Binding to the double-stranded oligodeoxynucleotide d (AAGAATTCTT)2 shows both 1:1 and 1:2 stoichiometries, with similar affinity to that of palmatine, and greater than those of coptisine or berberine (absolute values were not determined) (193). Similar studies with different sequences indicated that the affinity of jatrorrhizine (15) was reduced for d(AAGGATCCTT)2 and d(AAGCATGCTT)2 relative to the other protoberberine alkaloids tested (194). Finally, using competitive ethidium bromide displacement experiments on calf thymus DNA and synthetic double-stranded polynucleotides, the higher affinity of jatrorrhizine (15) relative to palmatine and berberine and their preference for AT-rich DNA were confirmed (195). In an eukaryotic test model (Euglena gracilis vs. the direct-acting mutagen acridine orange), jatrorrhizine (15) exhibited weak antimutagenic activity (196). Jatrorrhizine (15) was shown to be a weak scavenger of DPPH radicals, and a modest inhibitor of lipid peroxidation in unilamellar dioleyl-phosphatidylcholine liposomes (197). It downregulates tumor necrosis factor alpha (TNFa) and E-selectin expression, and decreases the content of thromboxane B(2) in rat intestinal microvascular endothelial cells, suggesting that it might reduce inflammatory response by affecting cytokines and autacoids (198,199), rather than by virtue of its poor antioxidant properties. Single doses of 50 and 100 mg/kg jatrorrhizine (15) decreased blood glucose in normal and alloxan-diabetic mice and increased succinate dehydrogenase activity in the liver, however, it had no effect on blood Author’s personal copy Alkaloids from the Genus Duguetia 139 lactic acid or liver lactate dehydrogenase. The alkaloid also decreased liver glycogen in normal mice, suggesting that its hypoglycemic activity can be attributed to increased aerobic glycolysis (184). Several methods have been used to study the binding of jatrorrhizine (15) to human serum albumin, concluding that the protein’s secondary structure is altered and hydrophobic and electrostatic interactions play a major role (200). Pseudopalmatine (17) does not seem to have been studied pharmacologically. E. Glaziovine (19) In the early 1970s the pharmacology of glaziovine (19) was explored by an Italian pharmaceutical company that registered it as a tranquilizer under the trademark Suavedols. Its psychopharmacology was compared with that of diazepam in a double-blind clinical trial (201), and its human pharmacokinetic parameters were studied (202). In addition, it was reported to possess anti-gastric ulcer properties in rodents and in humans (203,204). No studies appear to have addressed its mechanisms of action as either an anxiolytic or antiulcerogenic agent. More recently, glaziovine (19) was evaluated for anti-hepatitis B virus activity. This alkaloid proved to be highly potent, as judged by its IC50 value of 8 mM, as an inhibitor of HBV surface antigen production. The corresponding value for the positive control, the anti-HBV drug 3TC or Lamivudine, was 11.7 mM. However, glaziovine (19) was more toxic to uninfected that to infected cells (205). The isolated yield of glaziovine (19) from D. vallicola leaves was 0.27%, placing this abundant and easily accessible material in a good position as a source of a useful plant drug (26). Glaziovine (19) is one of 60 alkaloids listed as having particular pharmaceutical and biological significance (206). F. Aporphines Anonaine (3) relaxes rat aorta and tail artery precontracted with noradrenaline, predominantly through adrenergic receptors. Since its affinity for L-type Ca2þ channels is an order of magnitude less for a1 adrenoceptors in rat cerebral cortical membranes, it does not contribute to intracellular mobilization of Ca2þ, and its effect on phosphodiesterases is negligible. It is also slightly selective for a1A and a1D adrenoreceptors relative to the a1B subtype, as determined by radioligand competition experiments (207,208). Xylopine (28) is a selective a1 (vs. a2) adrenergic receptor antagonist with submicromolar functional potency (209). In the rabbit oviduct, isocorydine (41) inhibits spontaneous and noradrenaline-elicited Author’s personal copy 140 Edwin G. Pérez and Bruce K. Cassels contractions, indicating that this alkaloid is an adrenoceptor antagonist (210). A further study in a rat aorta model suggested that the effect is mediated primarily through a1 adrenoceptors (211). The effects of isocorydine (41) on the action potentials of canine heart muscle cells have also been studied in vitro (212). Asimilobine (20) inhibits rabbit aortal contractions induced by 10 6 M serotonin with pA2¼5.78, suggesting that this alkaloid is a 5-HT2 serotonin receptor antagonist (213). Dicentrine (39) inhibits the contraction of rat stomach muscle strips induced by serotonin, histamine, Kþ, and Ca2þ in a noncompetitive manner. In the case of serotonin-induced contractions, the relaxation depends on Ca2þ release from intracellular stores, suggesting that 5-HT (presumably 5-HT2B) receptors are involved (214). Asimilobine (20), nornuciferine (22), and anonaine (23) bind to 5-HT1A serotonin receptors with low micromolar IC50 values versus [3H] rauwolscine, and were shown to be full agonists (215). In [3H]8-hydroxy2-(di-N-propylamino)tetralin displacement experiments, N-methyllaurotetanine (37) exhibits high affinity for 5-HT1A receptors (Ki¼85 nM, pKi¼7.07) (216). Isoboldine (38) relaxes isolated guinea pig trachea with IC50¼710 mM, suggesting a b-adrenoceptor-mediated mechanism (217). Dicentrine (39) has been extensively studied as a cardiovascular agent. It was first shown to be a potent a1-adrenoceptor antagonist (less potent than prazosin, and more potent than phentolamine) with little effect on b-adrenergic receptors (218,219). Its hypotensive effect was demonstrated in vivo in rats by the intravenous and oral routes, and in conscious, spontaneously hypertensive animals, oral administration of 5 and 8 mg/kg caused hypotension lasting more than 15 h (220). In rats fed a high-cholesterol diet, oral administration of dicentrine (39) decreased the mean arterial pressure (more so in spontaneously hypertensive animals), and reduced the total plasma cholesterol by reducing the low-density lipoprotein fraction, and the total plasma triglyceride by reducing the very lowdensity lipoprotein fraction (221). Experiments in isolated cardiac cells and in rabbit heart showed that dicentrine (39) blocks sodium and potassium currents, and is a potentially useful antiarrhythmic agent at doses in the same range as quinidine (222,223). The effects of dicentrine (39) on the mechanical properties of systemic arterial trees have been studied in dogs (224). Dicentrine (39) inhibits serum-stimulated kidney mesangial cell proliferation in the rat, and was therefore viewed, together with other vasodilators, as an agent with the potential to delay the progression of chronic glomerulopathy (225). As an a1-adrenoceptor antagonist it also inhibits contractions of human hyperplastic prostate elicited by adrenergic stimulation, and might therefore be of use to relieve bladder outlet obstruction in patients with benign prostatic hyperplasia (226). Author’s personal copy Alkaloids from the Genus Duguetia 141 Anonaine (23) and isopiline (24) inhibit dopamine uptake by rat striatal synaptosomes with IC50¼0.8 and 2.5 mM, respectively. Anonaine (23) is a selective uptake inhibitor relative to its affinities for D1-like and D2-like dopamine receptors as determined in radioligand displacement experiments (IC50 vs. [3H]SCH23390 and [3H]raclopride: 68 and 19 mM, respectively; ratios of uptake to receptor binding IC50 values: 85.0 and 23.5), while isopiline (24) exhibits much lower selectivity (D1-like and D2like binding IC50: 10 and 34 mM, respectively; IC50 ratios 3.0 and 13.6) (227). Asimilobine (20), in the 0.05 0.2 mM range, reduces intracellular dopamine in PC12 cells for 24 h with IC50¼0.13 mM. At this concentration it decreases the activities of tyrosine hydroxylase (TH, by 73.2% and for a longer time) and aromatic L-amino acid decarboxylase, and reduces TH mRNA and intracellular cAMP levels. Alone, it does not alter PC12 cell viability at concentrations up to 5 mM. However, in association with L-DOPA asimilobine (20) inhibits the L-DOPA-induced increase in dopamine levels and enhances L-DOPA cytotoxicity (228). N-Methylasimilobine (21) is a significant inhibitor of platelet aggregation elicited by collagen, arachidonic acid (AA), and plateletactivating factor (PAF). Xylopine (28) and N-methyllaurotetanine (37) inhibit platelet aggregation with different potencies depending on the substance used as an aggregation inducer in each case (229). Dicentrine (39) also inhibits platelet aggregation induced by AA, collagen, adenosine diphosphate (ADP), PAF, thrombin, or the synthetic U46619, and induces ATP release from platelets. Additional experiments indicated that these effects are due to the inhibition of thromboxane B2 formation and increased cAMP levels (218,230,231). N-Methyllaurotetanine (37), administered intravenously, is antihyperglycemic in normal and streptozotocin-induced diabetic rats (232). N-Methyllaurotetanine (37) and norisocorydine (40), at 20 mg/kg i.p., are significantly antinociceptive in the acetic acid-induced mouse writhing test, and quench DPPH radicals with SC50¼28 and 14 mg/mL (82 and 43 mM), respectively (25,120). Antinociceptive activity is often associated with free radical inactivation, and in this regard it should be mentioned that anonaine (23) was one of the first aporphine alkaloids for which antioxidative activity was demonstrated (233). Anonaine (23) reduces the viability of normal rat hepatocytes, and HepG2 and HeLa tumor cells, with IC50 values of 70.3, 33.5, and 24.8 mg/mL, respectively, in 24-h experiments (234). Non-cancer Vero and MDCK cells exposed to 100 mM anonaine (23) for 24 h experienced reduced viability by about 25% and 5%, respectively (235). In the case of HeLa cells, the decrease amounted to 77%, and was associated with DNA damage and a dose-related block of the cell cycle before the G1 phase. These effects were correlated to increased intracellular nitric oxide, Author’s personal copy 142 Edwin G. Pérez and Bruce K. Cassels reactive oxygen species, glutathione depletion, disruption of the mitochondrial transmembrane potential, activation of caspases 3, 7, 8, and 9, and poly(ADP-ribose) polymerase (PARP) cleavage with upregulation of Bax and p53 proteins (235). Dicentrine (39) inhibits the growth of murine leukemia P388 and L1210, melanoma B16, bladder cancer MBC2, and colon cancer Colon 26 cells in culture, and also reduces mitogen-induced lymphocyte proliferation and the growth of IL-dependent CTLL2 cells (236). It slows the growth of the human hepatoma cell line HuH-7 and decreases the efficiency of colony formation by these cells and the MS-G2 line, and strongly inhibits DNA and RNA synthesis. Additional evaluations in 21 tumor cell lines showed that dicentrine (39) was particularly cytotoxic to esophageal carcinoma HCE-6, lymphoma Molt-4 and CESS, leukemia HL60 and K562, and hepatoma MS-G2 (237). This alkaloid is active in a DNA unwinding assay, and is a modest inhibitor of topoisomerase II (IC50¼27 mM) (238). However, it shows no antiproliferative activity versus several yeast strains (239). Duguetine (76) ‘‘caused considerable antitumoral activity’’ (240). An extract of D. odorata was found to inhibit the G2 DNA damage checkpoint, a target that is expected to enhance the effectiveness of DNA-damaging anticancer therapy. Dehydrodiscretine (16), pseudopalmatine (17), oliveroline (60), and N-methylguatterine (66), were isolated by bioassay-guided fractionation following this bioactivity, however, only oliveroline (60) had confirmed, though modest, potency (at concentrations above 10 mM), and was isolated in sufficient amounts for additional testing (14). Pachystaudine (82) interferes with the replicative cycle of herpes simplex virus type 1 (HSV-1) (241). Anonaine (23) and xylopine (28) are weakly antibacterial and antifungal (120,242,243), and anolobine (27) is only active against Gram-positive bacteria and Mycobacterium phlei in the 10 4 molar range with MIC90¼12 50 and 6 25 mg/mL, respectively (243). Anolobine (27) induces chromosomal aberrations in a Chinese hamster lung cell line at concentrations as low as 2.5 mg/mL (244). At 300 mg/mL, dicentrine (39) showed ‘‘moderate’’ to ‘‘good’’ activity against the fungi Microsporum canis, Microsporum gypseum, Trichophyton mentagrophytes, and E. floccosum, but was inactive against C. albicans, Aspergillus niger, and Penicillium sp. (245). Nornuciferine (22) and xylopine (28) are significantly active against Leishmania mexicana and Leishmania panamensis, with the latter alkaloid showing LD50¼3 mM, vs. L. mexicana, and 37-fold higher toxicity towards the parasite than towards the host cells, the macrophages (246). Dicentrine (39) is active against Trypanosoma brucei brucei in vitro with IC50¼3.15 mM (247). Duguetine (76) is moderately active against the trypomastigote form of Trypanosoma cruzi (IC50¼9.32 mM) (120). Author’s personal copy Alkaloids from the Genus Duguetia 143 Asimilobine (20), anonaine (23), xylopine (28), isolaureline (29), and dicentrine (39) are antiplasmodial at low-to-micromolar concentrations against the chloroquine-sensitive P. falciparum D-2 strain and the resistant W-2 strain, but under the same conditions chloroquine has 1.3 and 11.2 nM ED50 values against the sensitive and the resistant strains, respectively (248). Isocorydine (41) is moderately active in vitro against P. falciparum, with IC50¼37 mM, and practically noncytotoxic and inactive against E. histolytica (193). Oliveroline is active against P. falciparum at low micromolar concentrations (27). Dicentrine (39) reduces the motility of Haemonchus contortus larvae (the large stomach worm of ruminants), with EC90¼6.3 mg/mL, and an oral dose of 25 mg/kg in mice reduced the worm count by 67% (249). G. Oxoaporphines Atherospermidine (86) relaxes uterine contractions induced by high Kþ and by oxytocin, with a mechanism involving Ca2þ entry and release from intracellular stores (250). Liriodenine (83), in the 10 7 10 4 M range, relaxes rat aorta contracted with potassium chloride or norepinephrine, but in Ca2þ-free medium it does not inhibit the response elicited by caffeine, indicating that its vasorelaxant action is mediated by interaction with a1 adrenergic receptors and voltage-operated calcium channels (251). Dicentrinone (91) was also shown to possess weak vasorelaxant activity (252). Liriodenine (83) appears to regulate dopamine biosynthesis in the 5 10 mM range by reducing TH gene expression and activity, and is protective against L-DOPA-induced cytotoxicity in PC12 cells (253). At 100 mM liriodenine (83) inhibits platelet aggregation, particularly that elicited by ADP or collagen, and less by AA or PAF, with aggregation falling to 5.4%, 5.3%, 40.5%, and 84.1% of controls, respectively (229,252). Lanuginosine (87) shows similar activity to liriodenine (83) (254). Liriodenine (83) is cytotoxic to KB, A-549, HCT-8, and L-1210 tumor cells (255,256). It is also a mutagen for Salmonella typhimurium TA100 (257). Chromosomal aberrations are induced by liriodenine (83) at 5 mg/mL (244). Liriodenine (83) is selectively toxic against DNA repair- and recombination-deficient yeast mutants (IC12¼16.7 mg/mL vs. the rad 52 mutant), a model in which lysicamine (84) and O-methylmoschatoline (85) are inactive. The selectivity of liriodenine (83) suggested that its activity might be mediated by topoisomerase inhibition (258). Topoisomerase II inhibition by liriodenine (83) was confirmed in CV-1 cells infected with simian virus 40 (SV40), and it was also shown that this alkaloid is not a substrate for the verapamil-sensitive drug efflux pump (a mechanism underlying drug resistance) in CV-1 cells (248). Liriodenine Author’s personal copy 144 Edwin G. Pérez and Bruce K. Cassels (83) exhibits moderate antiproliferative activity versus the human breast cancer cell lines MCF-7, the doxorubicin-resistant MCF-7/ADR, and the estrogen receptor-deficient MDA-MB435 and MT-1 lines, with IC50¼15.6, 16.7, 16.4, and 18.2 mM, respectively (259). In another study versus MCF-7, NCI-H460, and SF-268 cell lines, IC50 values of 3.19, 2.38, and 2.19 mg/mL, respectively, were recorded (260). It should be pointed out that 3.19 mg/ mL corresponds to 11.6 mM, in good agreement with the earlier value. In A594 human lung cancer cells, liriodenine suppresses proliferation doseand time-dependent in the 2 20 mM range, mainly through cell cycle inhibition (G2/M arrest) and induction of apoptosis (261). Human hepatoma cell lines bearing the wild-type p53 oncogene (Hep G2 and SK-Hep-1) have also been challenged with liriodenine (83), which induced cell cycle arrest in the G1 phase and inhibited DNA synthesis, increasing the expression of p53 and inducible nitric oxide synthase, and the intracellular NO level (262). Lysicamine (84) is a modest inhibitor of the proliferation of two human liver cancer cell lines (Hep G2 and Hep 2,2,15) with IC50¼8.4 and 3.4 mg/mL, respectively (56). Dicentrinone (91) showed selective antiproliferative activity against some yeast strains, but not others. When tested against recombinant human topoisomerase I it only inhibited the enzyme to a small extent, stabilizing the enzyme DNA binary complex (239). Apparently, the earliest recorded biological activities of liriodenine (83) are antibacterial and antifungal, which it shares with lysicamine (84) (243,263,264). When mice infected with a lethal dose of C. albicans were treated with liriodenine (and also its methiodide), the proliferation of the pathogen was reduced significantly (265). The moderate activity of liriodenine (83) and O-methylmoschatoline (85) was demonstrated again more recently against several different fungi and bacteria (266,267). Liriodenine (83) was claimed to be a fairly potent growth inhibitor of Leishmania major and Leishmania donovani, showing inhibition at 3.12 mg/ mL (11.3 mM) (268), although another group reported IC50¼26.16 mM for a possibly different strain of L. donovani (269). A more recent study using Leishmania brasiliensis and Leishmania guyanensis promastigotes gave IC50¼58.5 and 21.5 mM, respectively, with O-methylmoschatoline (85) being about five times less active (270). Lysicamine (84) is also active against L. mexicana (245). Dicentrinone (91) is reported to have unusually potent leishmanicidal activity (IC50¼0.01 mM) (240). O-Methylmoschatoline inhibits the growth of Trypanosoma brucei at 6.25 mg/mL (268). Liriodenine (83) is active against P. falciparum with IC50¼15 mM (269,271). H. Aminoethylphenanthrenes Atherosperminine (94) produces behavioral stereotypy, increased spontaneous motor activity and amphetamine toxicity, reversal of Author’s personal copy Alkaloids from the Genus Duguetia 145 haloperidol-induced catalepsy, inhibition of conditioned avoidance response, inhibition of morphine analgesia, and potentiation of the anticonvulsant action of diphenylhydantoin, effects associated with dopamine receptor stimulation (272). It also inhibits the contraction of guinea pig trachealis muscle elicited by carbachol, prostaglandin F2a, a synthetic thromboxane analogue and leukotriene C4, it potentiates tracheal relaxation and cAMP accumulation elicited by forskolin and, at higher concentrations, by itself raises the content of cAMP, but not cGMP, in the tissue. Thus, its major mechanism of action seems to be the inhibition of cAMP phosphodiesterase (273). At 100 mg/mL, atherosperminine (94) and its N-methyl quaternary salt completely inhibited platelet aggregation elicited by ADP, AA, collagen, or PAF, while atherosperminine N-oxide (95), though inhibiting AA- and collagen-induced aggregation, is less effective against aggregation elicited by ADP or PAF. At this dose, atherosperminine (94) and its N-oxide are also complete antagonists of high potassium or norepinephrine-induced contractions of rat thoracic aorta, pointing to simultaneous a1-adrenoceptor and calcium channel inhibition (274). I. Copyrine Alkaloids Sampangine (97) potently inhibits HL-60 human leukemia cell proliferation by 50% at IC50¼2.65 mM, and its (lethal) DC50 value is 24.5 mM, suggesting that apoptosis plays a role in the cytotoxicity of this alkaloid, as confirmed by its effect at 20 mM on caspase-3 activity. At 4.0 mM sampangine induces cell cycle arrest in the G0/G1 phase, and at 20 mM leads to accumulation of cells with decreased DNA, typical of apoptotic cells. Low and high concentrations of sampangine (97) caused opposite effects on the potential of mitochondrial membranes, leading first to hyperpolarization (275). Treatment of HL-60 cells with sampangine (97) induced the rapid formation of reactive oxygen species, and quenching these with antioxidants abolished the pro-apoptotic activity of the alkaloid, indicating that sampangine-induced oxidative stress plays a key role in DNA damage (276). Sampangine (97) strongly inhibits the proliferation of human malignant melanoma cells (SK-MEL) with IC50¼0.37 mg/mL but, as observed previously in the HL-60 model, it is at least ten times less potent than other human cancer cells in culture (KB, BT-549, and SK-OV-3) (16). Hadranthine A (99) was practically inactive against the human cancer cells tested, but hadranthine B (100) inhibited the proliferation of SK-MEL, KB, BT-549, and SK-OV-3 cells with IC50¼3.0, 6.4, 6.6, and 3.6 mg/mL, respectively. Imbiline-1 (101) inhibited SK-MEL and SK-OV-3 cells with IC50¼2.0 and 5.0 mg/mL, respectively, but showed IC50 values greater than 10 mg/mL in the other cell lines (16). Author’s personal copy 146 Edwin G. Pérez and Bruce K. Cassels Sampangine (97) and 3-methoxysampangine (98) exhibit antifungal and antimycobacterial potencies about one half of those of amphotericin B and rifampicin, with MIC in the 0.78 1.56 mg/mL range against C. albicans, Cryptococcus neoformans, Aspergillus fumigatus, and Mycobacterium intracellulare, somewhat higher than the data published previously by these authors for the 3-methoxy derivative (277,278). In Saccharomyces cerevisiae, sampangine (97) induces oxidative stress, and its antifungal activity is at least partially due to alterations in heme metabolism (279). Sampangine (97), 3-methoxysampangine (98), hadranthine A (99), and imbiline-1 (101), but not hadranthine B (100), exhibit antiplasmodial activity in vitro against P. falciparum (chloroquine-resistant clone W-2 and chloroquine-sensitive clone D-6). Although about ten times less potent than chloroquine against the D-6 clone, hadranthine A (99) shows reasonably good selectivity (selectivity index W40) versus Vero cells, while the other alkaloids are even less potent and less selective. On the other hand, sampangine (97) and 3-methoxysampangine (98) are more potent than chloroquine against the chloroquine-resistant W-2 clone (16). J. 1-Aza-9,10-anthraquinones Cleistopholine (104) inhibits the proliferation of Hep G2 and Hep 2,2,15 human hepatocarcinoma cell lines, with IC50¼0.22 and 0.54 mg/mL, respectively (56). It has modest antifungal and antimycobacterial activities with MIC against C. albicans, C. neoformans, A. fumigatus, and M. intracellulare of 12.5, 1.56, 100, and 12.5 mg/mL, respectively (277), and has also shown activity against mutant S. cerevisiae strains, Cladosporium cladosporioides, and Cladosporium sphaerospermum (54). Cleistopholine (104) inhibits the growth of P. falciparum at low micromolar concentrations (27). VII. CONCLUDING REMARKS The foregoing sections illustrate a cyclic trend that has been developing for a long time in natural products research, but which seems to take on specific features in studies on plant families that are traditionally seen as rich sources of alkaloids. In its initial century, from the isolation of morphine and quinine through mescaline, alkaloid chemistry was largely motivated by the desire to understand and to better apply the medicinal or biological properties of plant drugs. Later on, rapid advances in structure elucidation methodology and instrumentation led to an approach akin to the mountaineer’s ‘‘Why climb it? Because it’s there!,’’ while biosynthetic work remained more concerned with a quest for explanations. Over the last few decades a renewed interest in practical uses fired the development of bioassay-guided fractionation and a Author’s personal copy Alkaloids from the Genus Duguetia 147 preference for biosynthetic studies related to commercially or medically important alkaloids. In the meantime, organic and medicinal chemists developed synthetic methodology, and used alkaloid structural templates to generate new drugs, and fruitful collaborative efforts continue, both in the pharmaceutical industry and in academia. In the specific case of Duguetia, the identification of known alkaloids and the discovery of new structures have slowed considerably, while the pharmacology of some of the more widespread constituents has made surprising progress. But something seems to be lacking. It is most likely that there is an enormous wealth of ethnopharmacological knowledge risking oblivion and still waiting to be recorded. If alkaloid chemistry is to contribute to our understanding of the biology of the genus, it needs to address a wider range of species, particularly those belonging to unexplored or little-explored sections, and metabolic profiling should be applied to many of the plants that have already been studied as well as those that have not. Bioassay-guided fractionation has yielded some spectacular results, but what bioassays should be used? Easy antibacterial assays (unlike antifungal or antiparasitic assays) do not seem to have uncovered anything of interest in higher plants, and natural products chemists are not usually qualified to identify apparently arcane biological targets such as some of those now pursued by the pharmaceutical industry, or to set up the necessary tests, stressing the need to collaborate with pharmacologists. Although much is known about the pharmacology of some Duguetia alkaloids commonly found in other plants, the more characteristic alkaloids such as the 7-oxygenated and the 9,11-dioxygenated aporphinoids remain practically untouched. And what about structural modification or analog synthesis? It is hoped that these comments will stimulate discussion in the alkaloid chemical community and invigorate research, leading to both qualitative and quantitative leaps in productivity, and to novel approaches that will surely have unsuspected, but doubtless very valuable, results. 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